<PAGE>   1
                                                                    Exhibit 10.4


                                                             From the Office of:
                                  STANDARD FORM              Burns & Levinson
                          PURCHASE AND SALE AGREEMENT        125 Summer Street
                                                             Boston, MA 02110

1.       PARTIES AND MAILING ADDRESSES
         (fill in)

         This 26th day of July, 1999, Barry L. Solar and Robert L. Solar, as
         Trustees of 211 Second Avenue Realty Trust u/d/t dated March 8, 1988
         and recorded with Middlesex South District Registry of Deeds, and not
         individually hereinafter called the SELLER, agrees to SELL and Geltex
         Pharmaceuticals, Inc., a Delaware corporation hereinafter called the
         BUYER or PURCHASER, agrees to BUY, upon the terms hereinafter set
         forth, the following described premises: (the "Premises"):

2.       DESCRIPTION
         (fill in and include title reference)

         The land area of approximately 4.737 acres together with the
         improvements thereon containing approximately 19,200 square feet of
         building area known and numbered as 211 Second Avenue, Waltham,
         Massachusetts, as shown on the Plan annexed hereto as Exhibit A (as
         affected by the taking/layout

3.       BUILDINGS, STRUCTURES, IMPROVEMENTS, FIXTURES
         {fill in or delete)

         Included in the sale as a part of said premises are the buildings,
         structures, and improvements now thereon, and the fixtures belonging to
         the SELLER and used in connection therewith including, if any, all
         wall-to-wall carpeting, drapery rods, automatic garage door openers,
         venetian blinds, window shades, screens, screen doors, storm windows
         and doors, awnings, shutters, furnaces, heaters, heating equipment,
         stoves, ranges, oil and gas burners and fixtures appurtenant thereto,
         hot water heaters, plumbing and bathroom fixtures, garbage disposers,
         electric and other lighting fixtures, mantels, outside television
         antennas, fences, gates, trees, shrubs, plants, and, ONLY IF BUILT IN,
         refrigerators, air conditioning equipment, ventilators, dishwashers,
         washing machines and dryers;

4.       TITLE DEED
         (fill in)
         * Include here by specific reference any restrictions, easements,
           rights and obligations in party walls not included in (b), leases,
           municipal and other liens, other encumbrances, and make provision to
           protect SELLER against BUYER's breach of SELLER's covenants in
           leases, where necessary.

         Said premises are to be conveyed by a good and sufficient quitclaim
         deed running to the BUYER, or to the nominee designated by the BUYER by
         written notice to the SELLER at least seven days before the deed is to
         be delivered as herein provided, and said deed shall convey a good and
         clear record and marketable title thereto, free from encumbrances,
         except
           (a) Provisions of existing building and zoning laws;
           (b) Existing rights and obligations in party walls which are not the
               subject of written agreement;
           (c) Such taxes for the then current year as are not due and payable
               on the date of the delivery of such deed;
           (d) Any liens for municipal betterments assessed after the date of
               this agreement;***
           (e) Easements, restrictions and reservations of record, if any, so
               long as the same do not prohibit or materially interfere with the
               current use of said premises;
           (f) The leases referred to in Exhibit B and any monthly
               tenancy-at-will of Evergreen Solar for a period after the
               termination of its lease referred to in Exhibit B
           (g) Taking for the layout of Second Avenue

5.       PLANS

         If said deed refers to a plan necessary to be recorded therewith the
         SELLER shall deliver such plan with the deed in form adequate for
         recording or registration.

6.       REGISTERED TITLE

         In addition to the foregoing, if the title to said premises is
         registered, said deed shall be in form sufficient to entitle the BUYER
         to a Certificate of Title of said premises, and the SELLER shall
         deliver with said deed all instruments, if any, necessary to enable the
         BUYER to obtain such Certificate of Title.

7.       PURCHASE PRICE
         (fill in); space is allowed to write out the amounts if desired

         The agreed purchase price for said premises is Three Million, Two
         Hundred Thousand ($3,200,000.00) dollars, of which
         $   50,000.00     have been paid as a deposit prior hereto
         $  100,000.00     have been paid as a deposit this day and
         $
         $3,050,000.00     are to be paid at the time of delivery of the deed by
                           certified, cashier's, treasurer's or bankcheck(s)
                           drawn on Boston area banks and payable to Seller
                           without intervening endorsement other than that of
                           Buyer; or if Seller*
         $
         --------------------------
         $3,200,000.00     TOTAL

         *so requests, by wire transfer of immediately available federal funds
         **of Second Avenue made after said Plan
         ***and the lien for municipal betterments assessed prior to the date of
         this agreement in connection with the taking/layout of Second Avenue


<TABLE>
<S>                                                <C>
COPYRIGHT (C) 1979, 1984, 1986, 1 987, 1988, 1991  [LOGO]  All rights reserved. This form may not be copied or
        GREATER BOSTON REAL ESTATE BOARD                   reproduced in whole or in part in any manner whatever
                                                           without the prior express written consent of the Greater
                                                           Boston Real Estate Board.
</TABLE>

<PAGE>   2

8.       TIME FOR PERFORMANCE; DELIVERY OF DEED (fill in)

         Such deed is to be delivered at 10:00 o'clock A.M. on the 4th day of
         October, 1999, at the offices of Burns & Levinson LLP in Boston,
         Massachusetts unless otherwise agreed upon in writing. It is agreed
         that time is of the essence of this agreement.

9.       POSSESSION AND CONDITION OF PREMISE
         (attach a list of exceptions, if any)

         Full possession of said premises free of all tenants and occupants,
         except * , is to be delivered at the time of the delivery of the deed,
         said premises to be then (a) in the same condition as they now are,
         reasonable use and wear thereof excepted, and (b) not in violation of
         said building and zoning laws, and (c) in compliance with provisions of
         any instrument referred to in clause 4 hereof. The BUYER shall be
         entitled personally to inspect said premises prior to the delivery of
         the deed in order to determine whether the condition thereof complies
         with the terms of this clause.

         *those under the leases and tenancies referred to in clause 4(f) hereof

10.      EXTENSION TO PERFECT TITLE OR MAKE PREMISES CONFORM
         (Change period of time if desired).

         If the SELLER shall be unable to give title or to make conveyance, or
         to deliver possession of the premises, all as herein stipulated, or if
         at the time of the delivery of the deed the premises do not conform
         with the provisions hereof, then the SELLER shall use reasonable
         efforts to remove any defects in title, or to deliver possession as
         provided herein, or to make the said premises conform to the provisions
         hereof, as the case may be, in which event the time for performance
         hereof shall be extended for a period of thirty days. Provided,
         however, Seller shall not be obligated to expend more than $50,000.00
         (including attorneys' fees)**

11.      FAILURE TO PERFECT TITLE OR MAKE PREMISES CONFORM, etc.

         If at the expiration of the extended time the SELLER shall have failed
         so to remove any defects in title, deliver possession, or make the
         premises conform, as the case may be, all as herein agreed, or if at
         any time during the period of this agreement or any extension thereof,
         the holder of a mortgage on said premises shall refuse to permit the
         insurance proceeds, if any, to be used for such purposes, then any
         payments made under this agreement shall be forthwith refunded and all
         other obligations of the parties hereto shall cease and this agreement
         shall be void without recourse to the parties hereto.

12.      BUYER's ELECTION TO ACCEPT TITLE

         The BUYER shall have the election, at either the original or any
         extended time for performance, to accept such title as the SELLER can
         deliver to the said premises in their then condition and to pay
         therefore the purchase price without deduction, in which case the
         SELLER shall convey such title, except that in the event of such
         conveyance in accord with the provisions of this clause, if the said
         premises shall have been damaged by fire or casualty insured against,
         then the SELLER shall, unless the SELLER has previously restored the
         premises to their former condition, either
           (a) pay over or assign to the BUYER, on delivery of the deed, all
               amounts recovered or recoverable on account of such insurance,
               less any amounts reasonably expended by the SELLER for any
               partial restoration, or
           (b) if a holder of a mortgage on said premises shall not permit the
               insurance proceeds or a part thereof to be used to restore the
               said premises to their former condition or to be so paid over or
               assigned, give to the BUYER a credit against the purchase price,
               on delivery of the deed, equal to said amounts so recovered or
               recoverable and retained by the holder of the said mortgage less
               any amounts reasonably expended by the SELLER for any partial
               restoration.

13.      ACCEPTANCE OF DEED

         The acceptance of a deed by the BUYER or his nominee as the case may
         be, shall be deemed to be a full performance and discharge of every
         agreement and obligation herein contained or expressed, except such as
         are, by the terms hereof, to be performed after or to survive the
         delivery of said deed.

14.      USE OF MONEY TO CLEAR TITLE

         To enable the SELLER to make conveyance as herein provided, the SELLER
         may, at the time of delivery of the deed, use the purchase money or any
         portion thereof to clear the title of any or all encumbrances or
         interests, provided that all instruments so procured are recorded
         simultaneously with the delivery of said deed or, in the case of the
         existing mortgage on the premises, customary arrangements are made for
         the subsequent procuring and recording of ***

         Until the delivery of the deed, the SELLER shall maintain insurance on
         said premises as follows:
            Type of Insurance                      Amount of Coverage
         ***the discharge thereof
         (a) Fire and Extended Coverage         *$ As presently insured
         (b)

15.      INSURANCE
         *Insert amount (list additional types of insurance and amounts as
          agreed)

16.      ADJUSTMENTS
         (list operating expenses, if any, or attach schedule)

         Collected rents, water and sewer use charges****, and taxes for the
         then current fiscal year****, shall be apportioned, as of the day of
         performance of this agreement and the net amount thereof shall be added
         to or deducted from, as the case may be, the purchase price payable by
         the BUYER at the time of delivery of the deed. Uncollected rents for
         the current rental period shall be apportioned if and when collected by
         either party.

         **in exercising reasonable efforts hereunder, but such limit shall not
         apply to amounts necessary to pay real estate taxes due and to pay off
         any mortgage or other voluntary lien granted by Seller
         ****(to the extent not payable by tenants under the leases)


<PAGE>   3


17.      ADJUSTMENT OF UNASSESSED AND ABATED TAXES

         If the amount of said taxes is not known at the time of the delivery of
         the deed, they shall be apportioned on the basis of the taxes assessed
         for the preceding fiscal year, with a reapportionment as soon as the
         new tax rate and valuation can be ascertained; and, if the taxes which
         are to be apportioned shall thereafter be reduced by abatement, the
         amount of such abatement, less the reasonable cost of obtaining the
         same, shall be apportioned between the parties, provided that neither
         party shall be obligated to institute or prosecute proceedings for an
         abatement unless herein otherwise agreed.

18.      BROKER's FEE
         (fill in fee with dollar amount or percentage; also name of Brokerage
         firm(s))

         A Broker's fee for professional services of (i) $160,000 is due from
         the SELLER to Spaulding & Slye Collier International and (ii) of
         $64,000.00 is due from the Seller to Insignia ESG, Buyer's broker, if,
         as and when papers pass hereunder, the deed is recorded and Seller
         receives the full consideration, due hereunder and only in such event.

19.      BROKER(S) WARRANTY
         (fill in name)

         The Broker(s) named herein Spaulding & Slye Colliers International,
         and Insignia ESG warrant(s) that the Broker(s) is (are) duly licensed
         as such by the Commonwealth of Massachusetts.

20.      DEPOSIT
         (fill in name)

         All deposits made hereunder shall be held in escrow by Spaulding & Slye
         Colliers International as escrow agent subject to the terms of this
         agreement and shall be duly accounted for at the time for performance
         of this agreement. In the event of any disagreement between the
         parties, the escrow agent may retain all deposits made under this
         agreement pending instructions mutually given by the SELLER and the
         BUYER. ALL deposits shall be held in an interest bearing account and
         all interest shall be paid to Buyer at the time of closing unless
         Buyer*

21.      BUYER's DEFAULT; DAMAGES

         If the BUYER shall fail to fulfill the BUYER's agreements herein, all
         deposits made hereunder by the BUYER shall be retained by the SELLER as
         liquidated damages as Seller's sole and exclusive remedy both at law
         or in equity for any by default Buyer.

22.      RELEASE BY HUSBAND OR WIFE

         *defaults hereunder in which case such interest shall be paid to Seller

23.      BROKER AS PARTY

         The Broker(s) named herein join(s) in this agreement and become(s) a
         party hereto, insofar as any provisions of this agreement expressly
         apply to the Broker(s), and to any amendments or modifications of such
         provisions to which the Broker(s) agree(s) in writing.

24.      LIABILITY OF TRUSTEE, SHAREHOLDER, BENEFICIARY, etc.

         If the SELLER or BUYER executes this agreement in a representative or
         fiduciary capacity, only the principal or the estate represented shall
         be bound, and neither the SELLER or BUYER so executing, nor any
         shareholder or beneficiary of any trust, shall be personally liable for
         any obligation, express or implied, hereunder. The provisions of this
         Section 24 shall survive the closing and shall apply to this Agreement
         and shall also apply to and be deemed to*

25.      WARRANTIES AND REPRESENTATIONS
         (fill in); if none, state "none"; if any listed, indicate by whom each
         warranty or representation was made

         The BUYER acknowledges that the BUYER has not been influenced to enter
         into this transaction nor has he relied upon any warranties or
         representations not set forth in this agreement, except for the
         following additional warranties and representations, if any, made by
         either the SELLER or the Broker(s): NONE

         *be incorporated in and to apply to all documents now or hereafter
         executed by Seller in connection herewith and in connection with the
         transactions contemplated hereby.


<PAGE>   4


27.      CONSTRUCTION OF AGREEMENT

         This instrument, executed in multiple counterparts, is to be construed
         as a Massachusetts contract, is to take effect as a sealed instrument,
         sets forth the entire contract between the parties, is binding upon and
         enures to the benefit of the parties hereto and their respective heirs,
         devisees, executors, administrators, successors and assigns, and may be
         cancelled, modified or amended only by a written instrument executed by
         both the SELLER and the BUYER. If two or more persons are named herein
         as BUYER their obligations hereunder shall be joint and several. The
         captions and marginal notes are used only as a matter of convenience
         and are not to be considered a part of this agreement or to be used in
         determining the intent of the parties to it.

30.      ADDITIONAL PROVISIONS

         The rider attached hereto is incorporated herein by reference.


FOR RESIDENTIAL PROPERTY CONSTRUCTED PRIOR TO 1978, BUYER MUST ALSO HAVE SIGNED
            LEAD PAINT "PROPERTY TRANSFER NOTIFICATION CERTIFICATION"


NOTICE: This is a legal document that creates binding obligations. If not
        understood, consult an attorney.

  /s/ Barry L. Solar, Trustee           /s/ Robert L. Solar, Trustee
  ------------------------------------  ----------------------------------------
  SELLER (or spouse)                    SELLER

  /s/ Paul J. Mellett CFO
  ------------------------------------  ----------------------------------------
  BUYER                                 BUYER

         /s/ Alexander D. Dauria        /s/ Stephen J. Murphy
         ------------------------------------------------------------
                                    Broker(s}



--------------------------------------------------------------------------------

                       EXTENSION OF TIME FOR PERFORMANCE
                                                               Date ____________

    The time for the performance of the foregoing agreement is extended until
______________________ o'clock __M. on the _____________ day of __________ 19__,
time still being of the essence of this agreement as extended. In all other
respects, this agreement is hereby ratified and confirmed.

    This extension, executed in multiple counterparts, is intended to take
effect as a sealed instrument.


  ------------------------------------  ----------------------------------------
  SELLER (or spouse)                    SELLER


  ------------------------------------  ----------------------------------------
  BUYER                                 BUYER


         ------------------------------------------------------------
                                    Broker(s}

--------------------------------------------------------------------------------



<PAGE>   5

                      RIDER TO PURCHASE AND SALE AGREEMENT
                                 BY AND BETWEEN
                BARRY L. SOLAR AND ROBERT L. SOLAR, AS TRUSTEES
                   AND NOT INDIVIDUALLY, OF 211 SECOND AVENUE
              REALTY TRUST, u/d/t DATED MARCH 8, 1988, AS SELLERS,
                                      AND
                     GELTEX PHARMACEUTICALS, INC., AS BUYER

31.  REPRESENTATIONS AND WARRANTIES OF SELLER. Subject to all matters disclosed
     in any document delivered to Buyer by Seller prior to the expiration of the
     Due Diligence Period or on any Exhibit attached hereto, or other
     information disclosed in writing to Buyer by Seller after the date hereof
     and prior to the expiration of the Due Diligence Period and to all matters
     disclosed in any documents received by Buyer from anyone prior to the
     Closing and to all matters discovered by Buyer or as to which Buyer had
     knowledge at the time of Closing from any source or should have known based
     upon a physical inspection of the Premises for a transaction of this nature
     and amount and to all matters recorded and properly indexed at the
     Middlesex South District Registry of Deeds (all such matters being referred
     to herein as "Exception Matters"), Seller represents and warrants as
     follows:

     (a)  AUTHORITY. Seller is a trust duly formed and validly existing under
          the laws of the Commonwealth of Massachusetts and has all requisite
          power and authority to enter into this Agreement and the documents
          contemplated by Section 36, and to perform its obligations hereunder
          and thereunder. The execution and delivery of this Agreement and the
          documents contemplated by Section 36 have been duly authorized.

     (b)  NO CONFLICT. The execution and delivery of this Agreement and the
          consummation of the transactions contemplated hereunder on the part of
          Seller does not and will not conflict with or result in the breach of
          any terms or provisions of, or constitute a default under, or result
          in the creation or imposition of any lien, charge, or encumbrance upon
          any of the Premises or assets of the Seller by reason of the terms of
          any contract, mortgage, lien, lease, agreement, indenture, instrument
          or judgment to which Seller is a party or which is or purports to be
          binding upon Seller or which otherwise affects Seller or any of the
          Premises, which will not be discharged or released at or prior to
          Closing. No action by any federal, state or municipal or other
          governmental department, commission, board, bureau or instrumentality
          is necessary to make this Agreement a valid instrument binding upon
          Seller in accordance with its terms. The execution and delivery of
          this Agreement and the documents contemplated by Section 36 and the
          performance of the transactions contemplated hereby and thereby do not
          and will not contravene any provision of the declaration of trust of
          Seller, any judgment, order, decree, writ or injunction issued against
          Seller, or any provision of any law or governmental ordinances, rules,
          regulations, orders or requirements applicable to Seller, and do not
          require any consent or approval which has not yet been obtained.




                                       1
<PAGE>   6




     (c)  LEASES. There are no leases or occupancy agreements currently in
          effect which affect the Premises other than the two (2) leases
          referred to in Exhibit B, together with such additional leases
          approved or permitted pursuant to this Agreement or any tenancy at
          will agreement referred to in Section 4(F) (collectively, the
          "Leases"). The Leases are in full force and effect, without to
          Seller's knowledge, any default by Seller or any tenant, nor, to
          Seller's knowledge, are there any defenses, counterclaims, or offsets.
          Seller has not given or received any notice of default or any claim
          which remains uncured or unsatisfied, with respect to any of the
          Leases, and to the best of Seller's knowledge, there is no basis for
          any such claim or notice of default by any tenant. No leasing,
          brokerage or like commissions, fees and payments are due from Seller
          in respect of the Leases. None of the tenants has paid more than one
          month's rent in advance except for security deposits. All tenant
          improvements or work to be done, furnished or paid for by Seller, or
          credited or allowed to a tenant, for, or in connection with, the
          Premises pursuant to the Leases have been completed and paid for. The
          information regarding the Leases contained in Exhibit C is true,
          accurate and complete as of the date hereof.

     (d)  NO CONDEMNATION. Seller has not received any written notice of any
          pending and, to Seller's knowledge, there is no contemplated
          condemnation, eminent domain, or similar proceeding with respect to
          all or any portion of the Premises. The foregoing does not apply to
          takings already made.

     (e)  CONTRACTS. There are no construction, management, leasing, service,
          equipment, supply, maintenance or concession agreements in effect with
          respect to the Premises which would be binding on Buyer to which
          Seller is a party.

     (f)  COMPLIANCE. Seller has not received written notice of any existing
          violations of any federal, state, county or municipal laws,
          ordinances, orders, codes, regulations or requirements affecting the
          Premises which have not been cured.

     (g)  LITIGATION. There is no action, suit or proceeding pending against
          Seller, or, to Seller's knowledge, any action, suit or proceeding
          pending or threatened against or affecting the Premises.

     (h)  ENVIRONMENTAL MATTERS. Except as set forth below and in any
          environmental assessment or report delivered to Buyer, (i) neither
          Seller nor, to Seller's knowledge, any third party has engaged in the
          generation, use, manufacture, treatment, storage or disposal of or has
          released, any Hazardous Wastes or Hazardous Substances at, from, on,
          under or to the Premises in violation of law, and (ii) neither Seller
          nor, to Seller's knowledge, any third party has received any written
          notice from any governmental authority having jurisdiction over the
          Premises of any violation of any environmental law, regulation or
          ordinance with respect to the Premises. Notwithstanding the foregoing,
          Seller has informed Buyer that (i) tenant, LPL Alexandria, Inc., uses
          film processing and


                                       2
<PAGE>   7


          developing chemicals, (ii) tenant, Evergreen Solar, Inc., manufactures
          solar panels, and (iii) numerous prior tenants of the Premises may
          have used hazardous substances in connection with their business; and
          the acts of the present and prior tenants are excepted from the
          foregoing warranties. As used in this Agreement, the terms "HAZARDOUS
          SUBSTANCES" and "HAZARDOUS WASTES" shall have the meanings set forth
          in the Comprehensive Environmental Response, Compensation and
          Liability Act, as amended, and the regulations thereunder, the
          Resource Conservation and Recovery Act, as amended, and the
          regulations thereunder, and the Federal Clean Water Act, as amended,
          and the regulations thereunder, and such terms shall also include
          asbestos, petroleum products, radioactive materials and any regulated
          substances under any Federal, State or local environmental law,
          regulation or ordinance.

     (i)  FIRPTA. Seller is not a "foreign person" as defined in Section
          1445(F)(3) of the Internal Revenue Code.

32.  LIMITATIONS REGARDING REPRESENTATIONS AND WARRANTIES.

     (a)  Seller shall have no liability whatsoever to Buyer with respect to any
          Exception Matters.

     (b)  As used in this Agreement, the phrase "to Seller's knowledge", or
          words of similar import, shall mean the actual (and not constructive
          or imputed) knowledge, without independent investigation or inquiry,
          of Robert L. Solar, Richard L. Solar and Barry L. Solar. Seller
          represents that Robert L. Solar, Richard L. Solar and Barry L. Solar
          are the individuals with primary responsibility for the sale of the
          Property and Robert L. Solar is the individual with primary
          responsibility for overseeing the management and operation of the
          Property.

     (c)  Buyer represents that, as of the date hereof, it has no actual
          knowledge of any breach of any of Seller's representations and
          warranties.

33.  CONDITIONS PRECEDENT TO BUYER'S OBLIGATIONS. All of Buyer's obligations
     hereunder are expressly conditioned on the satisfaction at or before the
     time of Closing hereunder, or at or before such earlier time as may be
     expressly stated below, of each of the following conditions (any one or
     more of which may be waived in writing in whole or in part by Buyer, at
     Buyer's option):

     (a)  ACCURACY OF REPRESENTATIONS. All of the representations and warranties
          of Seller contained in this Agreement shall have been true and correct
          in all material respects when made, and shall be true and correct in
          all material respects on the date of Closing with the same effect as
          if made on and as of such date. If Seller notifies Buyer prior to the
          Closing that any of said representations and warranties are no longer
          true in all material respects, the Buyer shall have the right to
          terminate this Agreement by notice in writing to Seller, given no
          later




                                       3
<PAGE>   8



          than the first to occur of (i) the Closing or (ii) three (3) business
          days after such notice is given, in which event all deposits and all
          interest earned thereon shall be returned to Buyer. If Buyer does not
          so elect to terminate, then such representations and warranties shall
          be void and of no further force or effect and Seller shall have no
          liability whatsoever to Buyer with respect thereto.

     (b)  PERFORMANCE. Seller shall have performed, observed and complied with
          all covenants, agreements and conditions required by this Agreement to
          be performed, observed and complied with on its part prior to or as of
          Closing hereunder.

     (c)  DOCUMENTS AND DELIVERIES.

          (i)  All instruments and documents required on Seller's part to
               effectuate this Agreement and the transactions contemplated
               hereby shall be delivered to Buyer and shall be in form and
               substance consistent with the requirements herein.

          (ii) Seller represents that Seller has delivered to Buyer prior to or
               simultaneously with the execution of this Agreement (and Buyer
               acknowledges receipt of same) the following materials: (i) a copy
               of Seller's title insurance policy; (ii) all environmental
               studies and reports regarding the Premises that have been
               produced for Seller or are in Seller's possession; (iii) any
               survey of the Premises that has been produced for Seller or is in
               Seller's possession; and (iv) true and complete copies of the
               current leases, or other agreements granting occupancy rights to
               any person or entity. The materials referred to in this
               subsection 33(c)(ii) consist of the following:

               -    Stewart Title Insurance Policy No. O-9941-08861

               -    Haley & Aldrich Report dated March 1998 entitled "Release
                    Notification and Downgradient Property Status Opinion 200
                    West Street, Waltham, Massachusetts"

               -    Goldberg-Zoino & Associates, Inc. report dated March, 1988
                    entitled "Environmental Site Assessment, 211 Second Avenue,
                    Waltham, Massachusetts"

               -    Notification to Abutters under the Massachusetts Wetlands
                    Protection Act sent by Geltex with respect to 153 Second
                    Avenue

               -    Survey dated April 21, 1988 prepared by Vanasse Hangen
                    Brustlin, Inc.

               -    Lease dated June 1, 1994 to LPL Alexandria, Inc., with
                    Addenda and Addendum 2, and renewal notification dated June
                    10, 1999

               -    Lease dated September 15, 1995 to Evergreen Solar, Inc. with
                    Renewal Agreement dated July __, 1999.



                                       4
<PAGE>   9


34.  DUE DILIGENCE PERIOD; ACCESS; PURCHASE "AS IS".

     (a)  During the Due Diligence Period, Buyer, its agents, representatives,
          architects and lenders shall be entitled to enter upon the Premises
          from time to time (such entry to be in the presence of Seller or
          Seller's agent and during regular business hours), including all
          leased areas, upon twenty-four (24) hours prior notice to Seller, to
          perform inspections and tests of the Premises, including surveys,
          environmental studies, examinations and tests of all structural and
          mechanical systems within the building, to examine the books and
          records of Seller relating to the Premises, and to communicate
          directly with the tenants at the Premises. After the Due Diligence
          Period, buyer, its agents, representatives, architects and lenders
          shall be entitled to enter upon the Premises from time to time (such
          entry to be in the presence of Seller or Seller's agent, and during
          regular business hours), including all leased areas, upon twenty-four
          (24) hours prior notice to Seller, for the purpose of preparing
          architectural designs and satisfying the requirements of Buyer's
          lender. Before entering upon the Premises, Buyer shall furnish to
          Seller evidence of general comprehensive and contractual liability
          insurance coverage of at least $2,000,000.00 and insuring against such
          risks as Seller may reasonably require. Such insurance shall name
          Seller as an additional insured. Notwithstanding the foregoing, Buyer
          shall not be permitted to unreasonably interfere with Seller's
          operations at the Premises or interfere with the tenants' operations
          at the Premises, and the scheduling of any inspections shall take into
          account the timing and availability of access to the tenants'
          Premises, pursuant to the tenants' rights under the Leases or
          otherwise. If Buyer wishes to engage in any testing or sampling of
          surface or subsurface soils, surface water, groundwater or any
          materials in or about the building in connection with Buyer's
          environmental due diligence, such testing or sampling shall be subject
          to Seller's prior approval (which approval shall not be unreasonably
          withheld) of a protocol for such testing or sampling which protocol
          shall detail the nature, timing, location and extent of such testing
          and sampling and the means and methods to be utilized. Buyer shall
          promptly repair at Buyer's sole cost and expense, any damage to the
          Premises caused by any such tests or investigations, shall indemnify
          and defend Seller from any and all loss, cost, liabilities, claims,
          and expenses whatsoever (including reasonable attorneys' fees of
          counsel selected by Seller) arising out of any damage to persons or
          property occurring in or about the Premises and caused in any way from
          the inspection permitted hereby, and shall promptly return the
          Premises to the same condition as it was in prior to the inspection.
          The foregoing indemnification shall survive Closing or the termination
          of this Agreement, notwithstanding any other provision of this
          Agreement.

     (b)  The term "Due Diligence Period", as used herein, shall mean the period
          ending at 5:00 p.m. on the date sixty (60) days after the date hereof.
          Buyer may, in its sole discretion, terminate this Agreement by giving
          written notice of such election to Seller on any day prior to and
          including the final day of the Due




                                       5
<PAGE>   10


          Diligence Period, in which event the Deposit, together with any
          interest earned thereon, shall be returned forthwith to Buyer and,
          except as expressly set forth herein, neither party shall have any
          further liability or obligation to the other hereunder. In the absence
          of such written notice, the contingency provided for in this Section
          34(b) no longer shall be applicable, and this Agreement shall continue
          in full force and effect.

     (c)  Buyer acknowledges and agrees that Buyer is acquiring the Premises in
          its "AS IS" condition, WITH ALL FAULTS, IF ANY, AND EXCEPT AS PROVIDED
          IN SECTION 31 HEREOF, WITHOUT ANY WARRANTY, EXPRESS OR IMPLIED. Other
          than as expressly set forth herein, neither Seller nor any agents,
          representative, or employees of Seller have made any representations
          or warranties, direct or indirect, oral or written, express or
          implied, to Buyer or any agents, representatives, or employees of
          Buyer with respect to the condition of the Premises, its fitness for
          any particular purpose, or its compliance with any laws or any other
          matter or thing affecting or related to the Premises, and Buyer is not
          aware of and does not rely upon any such representation. Buyer
          acknowledges that the Due Diligence Period will have afforded Buyer
          the opportunity to make such inspections (or have such inspections
          made by consultants) as it desires of the Premises and all factors
          relevant to its use, including, without limitation, the interior,
          exterior, and structure of all improvements, and the condition of
          soils and subsurfaces. Except as otherwise expressly set forth in any
          representation or warranty made by Seller in Section 31 of this
          Agreement, any information, documents or materials which have been or
          hereafter are made available to Buyer are made available solely as an
          accommodation to Buyer in the conduct of its due diligence, and Seller
          makes no representation or warranty as to the accuracy thereof. The
          provisions of this Section 34(c) shall survive Closing.

     (d)  If Buyer does not give notice to Seller terminating this Agreement
          within the time set forth above, the title to the Premises shall,
          except as to title matters occurring after the date of this Agreement,
          be conclusively deemed to comply with the requirements of Paragraph 4
          of this Agreement and, except as to matters occurring after the date
          of the Agreement, the condition of the Premises shall conclusively be
          deemed to comply with the requirements of paragraph 9 of this
          Agreement

35.  TERMINATION FOR DEFAULT. Notwithstanding anything in this Agreement to the
     contrary, Seller may cancel or terminate any of the Leases or commence
     collection, or other remedial action against any tenant without Buyer's
     consent upon the occurrence of any default by the tenant under said Lease,
     provided that Seller shall give written notice to Buyer of the same.

36.  SELLER DELIVERIES. At Closing, Seller shall deliver to Buyer the following,
     and it shall be a condition to Buyer's obligation to close that Seller
     shall have delivered the same to Buyer:



                                       6
<PAGE>   11


     (a)  A Massachusetts Quitclaim Deed ("Deed") to the Premises from Seller,
          duly executed and acknowledged by Seller.

     (b)  An Assignment and Assumption of Interest in Leases from Seller, duly
          executed by Seller.

     (c)  Originals or, if originals are not available, copies certified by
          Seller of the Leases.

     (d)  A notice to each tenant, executed by Seller, advising of the sale of
          the Premises and directing that rent and other payments thereafter be
          sent to Buyer at the address provided by Buyer at Closing.

     (e)  Such affidavits or letters of indemnity as the Buyer's title insurer
          shall reasonably require in order to issue, without extra charge, an
          owner's policy of title insurance free of any exceptions for unfiled
          mechanics' or materialmen's liens for work performed by Seller prior
          to Closing, or for rights of parties in possession other than pursuant
          to the Leases.

     (f)  A Non-Foreign Affidavit as required by the Foreign Investors in Real
          Property Tax Act ("FIRPTA"), duly executed by Seller.

     (g)  A certification by Seller that all representations and warranties made
          by Seller in Section 31 of this Agreement are true and correct in all
          material respects on the date of Closing, except as may be set forth
          in said certificate with respect to matters since the date hereof.
          Seller acknowledges and Buyer agrees that if said certificate does not
          satisfy Section 33(a) hereof, Buyer shall have the right to terminate
          this Agreement as its sole remedy and receive the Deposit and all
          interest accrued thereon.

     (h)  A Certificate of the Trustees of Seller and a Direction of the Sole
          Beneficiary of Seller evidencing approval of and authorization to
          execute this Agreement and the closing documents, and to consummate
          the transactions contemplated hereby.

     (i)  A Certificate of Legal Existence and Good standing for the limited
          partnership which is the sole Beneficiary of Seller.

     (j)  All other instruments and documents reasonably required to effectuate
          this agreement and the transactions contemplated hereby, including,
          without limitation, a closing statement describing the sources and
          uses of funds in connection with the Closing.



                                       7
<PAGE>   12


37.  BUYER DELIVERIES. At Closing, Buyer shall deliver to Seller the following
     and it shall be a condition to Seller's obligation to close that Buyer
     shall have delivered the same to Seller:

     (a)  A certification by Buyer that all representations and warranties made
          by Buyer in Section 40 of this Agreement are true and correct in all
          material respects on the date of Closing.

     (b)  The Assignment and Assumption of Interest in Leases referred to in
          Section 36(b) duly executed and acknowledged by Buyer.

     (c)  Duly certified resolutions from the Board of Directors of Buyer
          evidencing approval of and authorization to execute this Agreement and
          the closing documents, and to consummate the transactions contemplated
          hereby.

     (d)  A Certificate of Legal Existence for Buyer.

     (e)  All other instruments and documents reasonably required to effectuate
          this Agreement and the transactions contemplated hereby, including,
          without limitation, a closing statement describing the sources and
          uses of funds in connection with the Closing.

38.  NOTICES. All notices and other communications provided for herein shall be
     in writing and shall be sent to the address set forth below (or such other
     address as a party may hereafter designate for itself by notice to the
     other parties as required hereby) of the party for whom such notice or
     communication is intended:

                  If to Seller:     211 Second Avenue Realty Trust
                                    c/o Walcott Corporation
                                    1050 Commonwealth Avenue
                                    Boston, MA 02215

                  With a copy to:   Barry L. Solar, Esquire
                                    Bums & Levinson, LLP
                                    125 Summer Street
                                    Boston, MA 02110-1624

                  If to Buyer:      Mr. Paul Mellett
                                    Chief Financial Officer
                                    Geltex Pharmaceuticals, Inc.
                                    9 Fourth Avenue
                                    Waltham, MA 02154-7506



                                       8

<PAGE>   13

                  With a copy to:   Kathryn Cochrane Murphy, Esquire
                                    Palmer & Dodge
                                    One Beacon Street
                                    Boston, Massachusetts 02108

     Any such notice or communication shall be sufficient if sent by registered
     or certified mail, return receipt requested, postage prepaid; by hand
     delivery; by overnight courier service; or by telecopy, with an original by
     regular mail. Any such notice or communication shall be effective when
     delivered or when delivery is refused.

39.  BROKER'S REPRESENTATION. Each party warrants and represents to the other
     that such party has not dealt with any real estate broker in connection
     with this transaction or the Premises other than the brokers named in
     Paragraph 18 hereof, and each party agrees and does hereby indemnify and
     hold the other harmless from any liability or loss suffered or incurred by
     the other (including, without limitation, reasonable attorney's fees and
     court costs) arising out of a breach of this warranty and representation.
     Seller shall be responsible for any and all commissions and fees due to
     Spaulding & Slye Colliers International and for the commission of
     $64,000.00 due to Insignia ESG as set forth in Paragraph 18. Buyer shall be
     responsible for any other commissions and fees due to Insignia ESG. The
     provisions of this Paragraph shall survive delivery of the deed hereunder.

40.  REPRESENTATIONS OF BUYER.

     (a)  AUTHORITY. Buyer is a corporation, duly organized, validly existing
          and in good standing under the laws of Delaware and has all requisite
          power and authority to enter into this Agreement and to perform its
          obligations hereunder. The execution and delivery of this Agreement by
          Buyer has been duly authorized.

     (b)  NO CONFLICT. The execution and delivery of this Agreement and the
          consummation of the transactions contemplated hereunder on the part of
          Buyer does not and will not violate any applicable law, ordinance,
          statute, rule, regulation, order, decree or judgment, conflict with or
          result in the breach of any material terms or provisions of, or
          constitute a default under, or result in the creation or imposition of
          any lien, charge, or encumbrance upon any of the property or assets of
          the Buyer by reason of the terms of any contract, mortgage, lien,
          lease, agreement, indenture, instrument or judgment to which the Buyer
          is a party or which is or purports to be binding upon Buyer or which
          otherwise affects Buyer. No action by any federal, state, or municipal
          or other governmental department, commission, board, bureau or
          instrumentality is necessary to make this Agreement a valid instrument
          binding upon Buyer in accordance with its terms.



                                       9
<PAGE>   14


41.  RECORDING. This Agreement or any notice or memorandum hereof shall not be
     recorded in any public record. A violation of this prohibition shall
     constitute a material breach of Buyer, entitling Seller to terminate this
     Agreement.

42.  SURVIVAL. Unless otherwise expressly stated in this Agreement, each of the
     warranties and representations of Seller and Buyer shall survive the
     Closing and delivery of the Deed and other closing documents by Seller to
     Buyer, and shall not be deemed to have merged therewith; provided, however,
     that any suit or action for breach of any of the representations or
     warranties set forth herein must be commenced within six (6) months after
     the Closing, or any claim based thereon shall be deemed irrevocably waived.

     IN WITNESS WHEREOF, the parties have executed and delivered this Agreement
as of the date first above written.


                           SELLER:

                                    /s/ Barry L. Solar
                                    --------------------------------------------
                                    Barry L. Solar, as Trustee and not
                                    Individually, of 211 Second Avenue
                                    Realty Trust


                                    /s/ Robert L. Solar, Trustee
                                    --------------------------------------------
                                    Robert L. Solar, as Trustee and not
                                    Individually, of 211 Second Avenue
                                    Realty Trust


                           BUYER:

                                    GELTEX PHARMACEUTICALS, INC.


                                    By: /s/ Paul J. Mellett Jr.
                                        ----------------------------------------
                                    Name: Paul J. Mellett Jr.
                                          --------------------------------------
                                    Title: Chief Financial Officer
                                           -------------------------------------


                                       10
<PAGE>   15


                                   EXHIBIT B


1.      Lease dated September 15, 1995 to Evergreen Solar, Inc. as affected
        Addenda #1 dated September 15, 1995 by renewal agreement dated July __,
        1998;

2.      Lease dated June 1, 1994 to LPL Alexandria, Inc. (including Addenda and
        Addendum 2 as affected by letter dated June 10, 1999 from Lerner
        Processing Labs, Inc. exercising the option to renew contained in said
        lease and as further affected by letters from 211 Second Avenue Realty
        L.P. to Lerner Processing Labs dated respectively February 20, 1995,
        April 16, 1996 and March 29, 1999.



                                       1


<PAGE>   16

                                   Exhibit C


EVERGREEN SOLAR, INC.


Lease signing page         Dated September 14, 1995   Signed and dated

Renewal Agreement:         Dated July, 1998           Signed and dated

Actual rent being paid:    $11,750.00/mo.

Date July rent paid:       July 6, 1999

Expiration:                September 30, 1999

Security Deposit:          $5,875.00



LERNER PROCESSING LABS

Actual rent being paid:    $6,533.33

Date July rent paid:       June 28, 1999

Expiration:                March 31, 2005

Security Deposit:          $5,390.00




                                       1
<PAGE>   17


Amounts paid to Carl Cincotta Landscaping during 1998 for snow removal and
landscaping, January 1, 1998 through December 31, 1998.

Tenants were billed their share, plus 15% to cover cost of overhead.

         Evergreen Solar, Inc.               49%

         Lerner Processing Labs              51%

         Amount billed by Carl Cincotta:               $4,345.00



                                       2

<PAGE>   18





                          211 SECOND AVENUE REALTY LP
                               VENDOR QUICKREPORT
                         JANUARY THROUGH DECEMBER 1998


         TYPE                DATE             NUM              AMOUNT
-----------------------    -------           ----            ---------

Carl J. Cincotta
   Check                   1/14/98           1685              -200.00
   Check                    2/5/98           1688              -840.00
   Check                    3/3/98           1670              -150.00
   Check                    4/3/98           1673              -200.00
   Check                    5/5/98           1678            -1,135.00
   Check                    6/5/98           1681              -150.00
   Check                    7/7/98           1684              -530.00
   Check                   7/30/98           1687              -200.00
   Check                   9/10/98           1691              -245.00
   Check                   10/5/98           1695              -250.00
   Check                   11/9/98           1697               -95.00
   Check                   12/2/98           1700              -200.00
   Check                  12/15/98           1706              -150.00






                                       3