SOMEACCOUNT' ,-.-·OF.

.·,

THE ART OF

PH 0 TO .GEN ICDRAWING··.·.····
,,..

·.

..·

· · . ..

·.··.·OR THE PROCESS BY WHICH

NATURAL O.BJE_CTS MAY BE MADE TO DELI~EAT~THEMSELVES....

WITHOUT THE AID OF THE ARTIST'S PENCIL.

BY

tlENRY FOX Tf\.LBOT, EsQ.

(Read b~fore the Royal Society, January; SI, 1839.)

LONDON:PRINTED llY R. AND .T. E. TA.XL()R, REU LION · COURT, ELEI~T S'l'REBT; .

1839.

.

SOME ACCOUNTOF

THE ART OF PHOTOGENIC DRAWING.

~ I.

I_ - the spring of 1834 I began to put in practi_ce a method which I had devised ~ornetime preriou ly, for employing to put'poses of utility the very curious property which.
bas been lona known to c hemists to be possessed by the nitrnte of silver; namely,
its di.5colora ·on when expo_ed to t he nolet ra of light. This property appeared tome o be perhaps capable of useful application in the followi ng manner . .
I propo_ed to preild on a h eet of paper a ~uffic i ent quantity of the nitrate of silver,
and then to· set t~e p aper in the sunshine, having first placed before it some . object
casting a well-defined shadow. The light,, acting on the rest of the paper, would
naturally blacken it, while the parts in shadow would retain their whiteness. Thus I
expected that a kind of image 01· pictme would be proauced, resembling to a certain
degree the object from which it was derived. I expected, however, also, that it would
be necessary to preserve such images in a .p<?rtfolio, and to view them only hy candle.:
light; because if by daylight, the same natural process which formed the images
would destroy them, by blackening the rest of the paper.
Such was my leading idea before it was enlarged and corrected by experience. It
was not until some time after, and when I was in possession of several novel and.
curious results, that I thought of inquiring whether this process had been ever proposed or attempted before? I found that in fact it had; but apparently not followed
up to any extent, or with much perseverance. The few notices that I have been able
to meet with are vague and unsatisfactory; merely stating that such a method existsof obtaining the outline of an object, but going into no details respeeting the best
and most advantageous manner of proceeding. ··The only definite _a ccount of the matter which I have been able to meet with, is
contained in the fit-st volume of the Journal of the Royal Institution, page liO, fromwhich it appears that the idea was originally started by Mr. WEDGWOOD, and a _n umerous series of experimen.ts made both by him and Sir HUMPHRY DAVY, which however ended in failut'e. I will take the liberty of quoting a few pas~ages from thismemoir.
" The copy of a painting, immediately after being taken, must be kept in an obn

.ACCOUNT QF -THE ART OF PHOTOGENIC DRAWING.

It may indeed be examined in the shade, but in this case the exposure
only for a few minutes. No attempts that have been made to prevent the .
part.s from being acted upon by light, have as yet been successful. They .
been covered with a thin coating of fine varnish; but this has not destroyed
ceptibility of becoming coloured. When the solar rays are passed thro~gha print and thrown upon prepared paper, · the unshaded parts are slowly copied; but
e lights transmitted by the shaded parts are seldom so definite as to form a distinct
emblance of them by producing different intensities of colom.
" T he images formed by means of a camera obscura have been found to be too faintto produc~, in any moderate time, a n effect upon the nitrate of silver. To copy theseimages was th e first object of Mr. WEDGWOOD, but a~l his numerous experiments
p roved unsuccessful."
These a1·e th e observations of Sir HUMPHRY DAv v. I have been informed by ascientific friend t hat this unfavourable result of M r. W :EoawooD's and Sir HUMPHRY
D Avv's experiments, was the chief cause which discournged him from following up
with perseverance the idea which he had _also ~nte rtain ed of fixing the beautiful
images of the camera obscura. And no doubt, whe'n so -distinguished an experimenteras Sir HU MPHRY DAVY announced "that all experiments had proved unsuccessful,"such a statement was calculated materially to discourage furt her inquiry. The circumstance also, announced by DAVY, that th e paper on which these images were ·depicted was liable to become entirely dark, and that nothing· hither~o tried would
prevent it, would perhaps have induced me to consider th e attempt as hopeless, if I
had not (fortunately) before I read it, already discovered a method of_overcoming
this difficulty, and of fi.r:ing the image in such a manner that it is no more liable toinjury or destrnction.
In the course of my experiments directed to that end, I have been astonished atthe variety of effects which I ha.ve foun<l prodt1ced by a very limit~d number of diffei·ent processes when combined in val'ious ways ; and also at the length of time whichsometimes elapses before th e full effect of these man ifests itself with certainty. For
I have found that images form ed in this manner, whi ch h;we appeared in good p1;eservation at the end of twelve months from the time of t~eir form ation, have nevertheless somewhat altered dul'ing the· second year . . This circumstance, added to thefact that the first attempts which I made became indistinct in process of time (thepaper growing wholly dark), induced me to watch the progress of the change duringsome considerable time, as I thought that perhaps all these images would ultimately
be found to fade. away. I found, however, to my satisfaction, that this was not thecase; and having now kept a number of these drawiBgs during nearly five years
without their suffering any deterioration, I think myself authorized to drnw conclu-'
sions from rny expei'iments with more certainty. ·MR. TALBOT'S ACCOUNT OF THE AR'r OF PHOTOGENIC DRAWING.

5

~ 2.

Effect and Appearance <if these. Images.
The images obtained in this manner are themselves white, but the ground uponwhich they display themselves is variously and pleasingly coloured.
Such is the variety of which the proce.ss is· capable, that by merely varying the
proportions and some trifling details of manipulation, any of the following colours
are readily attainable:
Sky-blue,
Yellow,
Rose-colour,
Brown, of various shades,
Black. ·Green alone is absent from the list, With the exception Of a daL·k Shade of it, approaching to black. The blue-coloured variety has a very pleasing effect, somewhat
like that produced by the Wedgwood-ware, which has white figures on a blue ground.
This variety"also retains its colours perfectly if preserved in a portfolio, and not being
subject to any spontaneous change, requires no preserving process.
These different shades of colour are of course so many different chemical · compounds; or mixtures of such, which. chemists· have not hitherto distinctly noticed.
~ 3.

First Applicatfons of this Process:·'

The first kind of objects which I attempted to copy by this · process were flowers
arid leaves, either fresh or selected from my herbarium. These it renders with theutmost truth and fidelity; exhibiting even the venation of the l~aves, the minute hairsthat clothe the plant, &c. &c.
It is so natural to associate the idea of labour with great complexity and elaborate
detail of execution, that one is more struck at seeing the thousal).d florets of an
Agrostis depicted with all its capillary branchlets (and so· accurately, that none Of
all this multitude shall want its little bivalve calyx, requiring to be examined through ·a lens), th'!-n one is by the picture of the large and simple leaf of an oak or a chestnut. But in truth the diffi.cuity is in both cases the same. The one of these takes
no more time to execute than the other; for the object which would take the mostskilful artist days or weeks of labour to trace or to copy, is effected by the boundless
powers of natural chemistry in the sp.ace of a few se~onds.
·
To gi.ve an idea of the degree of accuracy with which some objects can be imitatedby this process, I need only mention one instance. Upon one occasion, having madean image. of a piece of lace of an elaborate pattern, .I showed it to some persons ·at
the distance of a few feet, with the ·inquiry, :w hether it was a good representation ?1
when the reply was, "That th ey were not to be · so easily deceived, for that it WC\~ ,evidently no picture, but the piece of lace itself."
,,
B2

6

. MR. TAJ,~OT's - ACCOUN'r OF THE ART OF_ PHOTOGENIC DRAWING.

At the very commei;icement of my experiments upon this subject, when I saw how
beautiful were the images which were thus produced by the action of light, I regretted_the more that they were destined to have such a brief existence, and I resolved to attempt to find out, if possible, some method of preventing this, or retarding it as ~uchas possible. The following considerations led me to conceive the possibility of discovering: a pt·eservative pr9cess.
The nitrate of silver, which has become black by the action of 'light, is no longei·
the same chemical s~bstance that it was before. Consequently, if a picture produced
by solar light is subjected afterwards to any chemical process, the white and dark
parts of it wi1l be differently acted upon; and there is no evidence that after this action has taken place, these white and dark parts will any longer be subject to a spontaneous change ; ·or, if they are so, still it does not follow that that change will now
tend to assimilate them to each other. In case of their remaining dissimilar, the picture will remain visible, and therefore our object will be accomplished .
If it should be asserted that exposurn to sunshine would necessarily reduce thewhole to one uniform tint, and destroy the pictme, the onus probandi evidently lieson those who make the assertion. If we designate by the letter A the exposure to
the solar light, and by B some indeterminate chemical process, my argument was
this: Since it cannot be shown, a priori, that the final result of the series of processes
A B A will be the same with that d~noted by B A, it
therefore be worth while toput the matter to the test of experiment, viz. by varying the proeess B until the rightone be discqvered, or until so many t1·ials have ~een made as to preclude all reason able hope of its existence.
My first trials were uns:Uccessfol, as indeed .I expected; but after some time I discovered a method which answers perfectly, an? shortly afterwards another. On- oneof these more especially I have made numerous experiments; the other I have comparatively little used, because it appears to require more nicety in the management.
It is, however, equal, if not superior, to the first in brilliancy of effect.
This chemical change, which I call the preserving process, is far niore effectual than
could have been anticipated. The paper, which had previo.usly been so sen!'iitive tolight, becomes completely insensible to it, insomuch t_h at I am able to show the Society specimens which have been exposed for an hour to the foll si1mmer sun, andfrom which exposure the image has suffered nothing, but retains its perfect whiteness.

will~ 4. · On the Art

offixing a Shadow.

The phenomenon ~hich I have now briefly mentioned appears to me to partake ofthe charncter of the marvellous, ali:i1ost as much as any fact which physical investigation has yet brought to our knowledge. The most transitory of things, a shadow,
the prov~rbial emblem of all that is fleeting -and momentary, may be fettered by thespells of . our "n.a tural magic," and may be fixed for ·e ver in the position which it
seemed only destined for a single instant to_occupy.

MR. TALBOT'S ACCOUNT OF THE ART OF PHOTOGENIC DRAWING.

7

This remarkable phenomenon, of whatever value it may turn out in its application
to the arts, will at least be accepted as a new proof of the value of the inductive methods of modern science, which by noticing the occurrence of unusual circumstances(which accident perhaps first manifests.in s.ome small degree), and by follo'wing themup with experiments, and varying the conditions of these until the true law of nature
which they express is apprehended, conducts us at length to consequences altogether- unexpected, remote from usual experience, and contrary to almost universal belief.
Such is the fact, that we may receive on paper the fleeting -shadow, arrest it there,
and in the space of a sing·le minute fix it there so firmly as to be no more c.apable ofchange, even if thrown back into the sunbeam from which it derived its or1gin.

§ 5.
Before going further, I may however add, that it is not always necessary to use apreserving process. This I did not discover until after I had acquired considerable
practice in this art, having supposed at first that all these pictures would ultimately
become indistinct if not preserved in ·some ·way from the change. But experiencehas shown to me that there are at least two or three different ways in which the process may be conducted, so that- the images shall possess a character of durability,
provided they are k ept from the acti'on of direct sunshine. These ways have presented
th.emselves to notice rather accidentally than otherwise; in some instances without
any particular memoranda having been made at the time, so that I am not yet prepared to state accurately on what particular thing this sort of semi-durability depends,
or what course is best to be fo1lowed in order to ·obtain it. But as I have found thatcertain of the images which have been subjected to no preserving process remain ·quite white and perfect after the lapse· of a year or two, and indeed show no symptom
whatever of changing, while others differently prepared (and left unpreserved) havegrown quite dark in one tenth of that time, I think this singulal'ity requires to be
pointed out. Whether it will be of much value I do not know; perhaps it will bethought better to incur at first the small additional trouble of einploying the preserving process, especially as the drawing·s thus prepared will stand the sunshine;
while the unpreserved ones, however well they last in a portfolio or in common daylight, should not be risked in a very strong light, as they would be liable to change
thereby, even years after their original formation. This very quality, bowever, admitsof useful application. -For this semi-durable paper, which retains its whiteness foryears in the shade, and yet sutlers a change whenever exposed to the solar light, isevidently well suited to the use of a naturali st travelling_in a _distant · country, who
may wish to keep some memorial of the plants he finds, without having the troubleof drying them and carrying them about wit_h him. He would only have to take asheet of this paper, throw the image upon it, and replace it in his portfolio. The defect of this particular paper is, that in -g enera_! the ground -is not even; but this is ofno consequence where utility alone, and not beauty of effect is consulted.

R. TAI.BOTS ACCOUN'l' OF THE ART OF PHOTOGENIC DRAWING.

§ 6. Portraits.
A.no her purpose for which I think my method wiU _be found very convenient, Is
the making of outline portraits, or silho"!ettes. These are now often traced by theband from shadows projected by a candle. But the hand .is liable to err from thetrue outline, and a very small deviation causes a notable diminution in the resemblance. I believe this manual process cannot be compared with the truth and fidelity
with which the portrait is given by means of solar light.

§ 7. Paintings on Glass.
The shadow-pictures which are. formed by exposing paintings on glass to solar lightarn very pleasir{g. The glass itself, around the painting, should be blackened; such,for instance, as are often employed for the magic lantern. The paintings on the glass ·should have no bright yellows or reds, for these stop the.violet rays of light, which are
the only effective ones. The ·pictures thus formed resemble the productions of theartist's pencil more, perhaps, than any of the others. Persons to whoin I have shown
them have generally mistaken them for such, at the same time observing, that thestyle was new to them, and must be one rather difficult to acquire. It is in thesepictures only that, as yet, I have observed indications of colour. I have not had time
to pursue this branch of the inquiry further. It would be a great thing if by anymeans we could accomplish the delineation of objects in their natural colours, I ·am
not very sanguine respecting the possibility of this _; yet, as I have just now remarked,
it appears possible to obtain 'at least some indication of variety of tint.

§ 8. Application to the Microscope.
I now come to a branch ~f the subject which appears to me very important andlikely to prove extensively useful, the · application of my method of delineating objects to the solar microscope.
The objects which the microscope unfolds to our view, curious and wonderful as
they are, are often singularly complicated. The eye, indeed, may comprehend thewhole which is presented to it in the field of view; but the powers of the pencil failto express these minutire of nature in their innumerable details. What artist could
have skill or patience. enough to copy them? or gmnting that he could do so, must
it not be . at the expense of much most valuable time, which might be mo1·e usefully
employed?
Contemplating the beautiful picture which the solar microscope produces, thethought stnlck me, whether it might not be possible to cau~e that image to impress
itself upon the paper, and thus to let Nature substitute her own inimitable pencil,
for the imperfect, tedious, and almost hopeless attempt of copying a subject so intricate.
My first attempt had no success. Although I chose a bright day, and formed a goodMR. TALBO'l"S ACCOUNT OF THE ART OF PHOTOGENIC DRAWING.

9

image of my object upon prepared paper. on returning at the expiration of an hour I
found that no effect had taken place. I was therefore half inclined to abandon thisexperiment, when it occurred to me, that there was no reason to suppose that either
the nitrate or muriate of silver; as commonly obtained, was the most sensitive sub.stance that exists to the action of the chemical rays*; and though such should eventually prove to be the fact, at any rate it was not to be assumed with.o ut proof. I
therefore began a course of experiments in orde1· to ascertain the influence of various
modes of preparation, and I found these to be signally different in their results. I
considered this matter chiefly in a practical point of view; for as to the theory, I
confess that I cannot as yet understand the reason why the paper prepared in one
way should be so much more sensitive than in another.
The result of these experiments was the discovery of a mode of preparation greatly
superior in sensibility to what I had originally employed: and by means of this, allthose effects which I had before only anticipated as theoretically possible were foundto be capable of realization.
. When a sheet of this, which. I shall call " Sensitive Paper," is placed in a darkchamber, apd the magnified image of · so me obje~t thrown on it by the sola1· microscope, after the lapse of perhaps a quarter of an hour, the picture is fou nd to becompleted. I have not as yet used high magnifying powe1·s, on account of _th e consequent enfeeblement of the light. Of course, with a more sensitive paper, gTeater
magnifying power will become desirable.
.
On examining one of these picture_s, which I made about three years and a halfago, I find, by actual measurement of the picture and the object, that the latter isniagnified seventeen times in linear diameter, and in surface consequently 289 times.
I have others which I believe are considerably more magnified; but I have lost thecorresponding objects, so that I cannot here state the exact numbers.
Not only does this process save our t im e and trouble, but there are many objects,
especially microscopic crystallizations, which alter so greatly in the course of .thl'ee orfour days (and it could hardly take any artist less to delineate them in all their details), that they could never be drawn in the usual way.
I will now describe the degree of sensitiveness which this paper possesses> premisingthat I am far from supposing that I have reached the limit of which this quality iscapable. On the contrary, considering the few ex.periments which I have made, (few,that is, in comparison with the number which it would be easy to imagine and propose) I think it most likely, that other methods may be found, by which substances
may be prepared, perhaps as much transcending in sensitiveness the . one which I
have employed, as that does th~ nitrate of silver which I used in my first experiments.
But to confine myself to what I have actually accomplished, inthe preparation of'·* Sir H. DAVY _somewhere says that the iodide is more sensitive, which_I have hardly found to be the casein my experiments.

10:M R. TALBOT'S ACCOUNT OF THE ART OF PHOTOGENIC DRAWING.

a Y-ery sens1t1ve paper. When a sheet of this paper is brought t~wards a window,
not one through which the sun shines, but looking in the opposite direction, it immediately _b egins to discolour. For this reason, if the paper is prepared by daylight,
it must by no means be left uncovered, but as soon as finished be shut up in a drawer
or cupboard and there left to dry, or else dried at night by the warmth of a fire.
Before using this paper for the delineation of any object, I generally approach it fora little time towards the light, thus intentionally giving it a slight shade of colour,
for the purpose of seeing that the ground is even. If it appears so when thus tried
to · a small extent, -it will gener3)ly be found to prove so in the final result. But if
there are · some places or spots in it which do not acquire the same tint as the rest,
such a sheet of paper should be rejected; for there is a risk that, when employed,
instead of presenting a ground uniformly dark, which is essential to the beauty of thedrawing, it will have large white spots, places altogether insensible to the eff~ct oflight. This singular circumstance 1 shall revert to elsewhere: it is sufficient to mention it here.
The paper then, which is thus readily sensitive to the light of a common window,of course much more so to the direct sunshine. Indeed, such is the velocity of the ·effect then produced, that the picture may be said to be ended almost as soon as it isbegun.
To give some more definite idea of the rapidity of the process, I will state, that after
various trials the nearest evaluation which I could make of the time necessary for
obtaining the picture of an object, so as to have pretty distinct outlines, when I
employed the full sunshine, was half a second.

is

~ 9.

Architecture, -Landscape, and e:x:fernal Nature.

But perhaps the most curious application of this art is the one I am now about torelate. At least it is that which has appeared the most surprising to those who haveexamined my collection of pictures formed by solar light.
Every one is acquainted with the beautiful effects which ai·e produced by a camera
obscura and has admired the vivid picture of external nature which it displays. It
had often occurred to me, that if it were possible to retain upon the paper the lovely
scene which thus illuminates it for a moment, or if we could but fix the outline of it,
the lights and shadows, divested of all colour, such a result could not fail to be mostinteresting. And however much I might be disposed at first to treat this notion as ascientific dream, yet when I had succeeded in fixing the images of the solar microscope by means of a peculiarly sensiti ve paper, there appeared no longer any doubtthat an analogous process would succeed in copying the objects of external nature,
although indeed they are much less illurriinated.
.
Not having with me in the country a camera obscura of any considerable size, I
constructed one _o ut of a large box, the image being thrown upon one end of it by agood object glass fixed in the opposite end. This apparatus being armed \Vith a sen-MR. TALBOT'S ACCO i IT OF THE ART OF PHOTOGE1 JC DRAWDIG.

11

sitive paper, was taken out in a u mmer afternoon and placed about one h undred ards
from a building farnurably illuminated by the sun. An hour or t wo afte l'wards I
opened the box, and I found depicted upon the paper a very distinct representati onof t he building, with the exception of .those parts of it which lay in the shade. A
little experience in th is branch of the art showed me that with smalle1· carnerre obscurre the effect would be produced in a smaller time. Accordi ng ly I had several smallboxes made, in which I fixed lenses of shorter focus, and with these I obtained very
pe rfec t bnt extremely sqiall pictures; such as without great stretch of imagination
might he supposed to be tb e work of some Lilliputian artist. Th ey require indeed
examination with a lens to discover all their minutire.
In the summer of 1835 I made in thi s way a great number of representations of
my house in the country, which is well suited to the purpose, from its ancien t andremarkable architecture. And this building I believe to be the first that was ever
yet known to have drawn its own picture.
The method of proceeding was thi s : having first adjusted the paper to the prope1·
focus in each of these lit ~le camerre, I then took a number of them with me out of
doo rs and placed them in different situations arnund the buildi ng. After the lapse ofhalf an hour I gathered them all up, and brnught them within doors to open th em.
When opened, th ere was found in each a miniature picture of the obj ects, before
which it had been placed.
To the trnvelle1· in distant land s, who is ignorant, as too many unfo rtunately are,of the a rt of drawing, this little inventi on may prove of real service; and even to
the artist him s P-lf~ ho wever skilful he may be. Fo r although this natural prncess
does not produ ce an effect much resembling the productions of his pencil, and therefore cannot be considered as capable of replacing- them, yet it is to be recollectedthat he may often be so situated as to be able to devo te only a single hour to thedelineation of some very interesting locality. Now, since nothing prevents him fromsimultaneously disposing-, in different positions, any number of these little camerre, itis evident that their collective results, when examined afterwards, may furni sh himwith a large body of interesting memorials, and with numerous de tails which he had
not had him self time either to note down or to delineate.
~ 10.

D elineations of Sculpture.

Another use which I propose to make of my invention is for the copying of statues
and bas-reliefs. I place these in strong sunshine, and put before the m at a propel'
distance, and in the requisite position, a small camera obscura containing the prepared paper. In this way I have obtained images of various stat ues, &c. I havenot pm:sued this branch of the subject to any extent; but I expect interesting resultsfrom it, and that it may be usefully employed under many circumstances.

12

MR. ·TALBO'I"S ACCOUNT OF THE ART OF PHOTOGENIC DRAWING.

~ 11.

Copying of Engravings.
The invention may be employed with great facility for obtaining copies of drawings or eng,ravings, or facsimiles of MSS. Fo1· this purpose the engraviug is pressed
upon the prepared paper, with its engraved side in contact with the latter. The
pressure must be as uniform as possible, that the contact may be perfect; for the
least interval sensibly injures t he result, by producing a kind of cloudiness in lieu ofthe sharp strokes of the original.
When placed in the sun, the solar light gradually traverses the paper, except in
those places where it is prevented from doing so by the opake lines of the engraving.
It therefore of course makes an exact image or print of the design. This is one of
the experiments which DAVY and WEDGWOOD state that they tried, but failed, fromwant of sufficient sensibility in their paper.
The length of time requi site for effecting the copy depends on the thickness of thepaper on which the engraving has been printed. At first I thought that it would not
be possible to succeed with thick papers; but I found on trial that the success ·of themethod was by no means so limited. It is enough for the purpose, if the paper allows
any of·the solar light to pass. When the paper is thick, I allow half an hour for theformation of a good copy. In this way I have copied very minute, c~Hnplicated, anddelicate engravings, crowded with figures of small size, which were rendered withgreat distinctness.
The effect of the copy, though of course unlike the original, (substituting as it does
lights for shadows, and vice versd,) yet is often very pleasing, and would, I think, suggest to artists useful ideas respecting light and shade.
It may be supposed that the engraving would be soiled 01· injured by being thuspressed against the prepared paper. There is not much danger of this, provided bothare perfectly dry. It may be well to mention, however, that in case any stain shouldbe perceived on the engraving, it may be readily removed by a chemical application
which does no injury whatever to the paper.
In copying engravings, &c. by this method, the lights and shadows are reversed,
consequently the effect is wholly altered. But if the picture so obtained is first preserved so as to bear sunshine, it may be aftenvards itself employed as an object to becopied; and by means of this second process the lights and shadows are broughtback to their original disposition. In thi s way we have indeed to contend with theirnperfections arising from two processes instead of one; but I believe this will befound merely a difficulty of manipulation. I propose to employ this for the purpose.
more particularly of multiplying at small expense copies of such rare or unique engravings as it would not be worth while to re-eng:rave, from the limited demand forthem.
I will now add a few remarks concerning the very singular circumstance, which I
have before briefly mentioned, viz. that the paper sometimes, although intended toMR . 'fr\.LBOT'S ACCOUNT OF THE ART OF PHOTOGENIC DRAWJNG.

13.be prepared of the most sensitive qua]ity, turns out on trial to be, wholly insensible
to Jight, and incapable of change. The most singular part of this is the very small
difference in the mode of preparation which causes so wide a discrepancy in the result. For instance, a sheet of paper is all prepared at the sam e time, aRd with theintention of giving it as much uniformity as possib]e: and yet, when exposed tosunshine, this paper will exhibit Jarg·e white spots of very definite outline, where thepreparing process has faiJed ; the rest of the paper, where it has succeeded, turning
black as rapidly as possible. Sometimes the spots are of a pale tint of crerulean
blue, and are surrounded by exceedingly definite outlines of perfect whiteness, contrasting very much with the blackness of the part immediately succeeding. With regard to the th eo ry of this, I am only prepared to state as my opinion at present, that
it is a case of what is called "unstable equilibrium." The process followed is suchas to produce one of two definite chemical compounds; and when we happen to comenear the limit which separates the two cases, it depends upon exceedingly small andoften imperceptible circumstances, which of t he two compounds shall be formed .
That they are both definite compounds, is of course at present merely my conjecture ;that they are signally different, is ev ident from their dissimilar properties.
I have thus endeavoured to give a brief outline of some of the peculiarities attending this new process, which I offer to the lovers of science and nature. That it issusceptibJe of great improvements, I have no manner of doubt; but even in its present
state I believe it will be found capable of many useful and important applications besides those of which I have given a short account in the preceding pages.

Printed by R. and J.E. Taylor, Red Lion Court, Fleet Street.