Green-Mountain freeman. [volume] (Montpelier, Vt.) 1844-1884, 1879-10-08
Scientific.
BRAZILIAN PEBBLES.

To the inquiry "what are Brazilian
pebbles, of which spectacle glasses are
made" we reply they are pure crystalliz-
ed silica, species of flint. They are not
necessarily all Brazilian; but as they have
been found plentifully and very perfect in
that country, demand has sprung up for
the supply of manufacturers in London,
Paris and New York. Spectacle lenses
were formerly all made of glass, but they
have been found to be much less pure,
and at the same time to be more heating
to the eye than the natural product of the
earth. The pebble is much harder than
glass, indeed will easily scratch glass, so
that while glass is very likely 10 be
scratched by the freqnent rubbing to which
spectacle lenses are subjected for the
purpose of cleaning them, pebble can
suffer no such injury. A still more i'm
portant fact is that glass is a mixture of
silica and some alkali, and this alkali has
strong affinity for oil or grease, with
which it combines to form soap; hence
holding polished glass in the fingers per-
manenty dims it. So does the oil from
the face that frequently is found on the
lenses of spectacles; but the pebble lenses
are lltDle W uO luJuly HUN lAaV o'Iuoo, 40
they contain no alkali
An examination of the Brazilian pebble
lenses with the tourmaline tongs, or ''peb-
Bile trier," shows common glass to be
almost dark, where the rock crystal is
luminous; and, on touching them both to
the tongue, there is observable at once a
most marked difference us to temperature.
Brazilian pebble spectacles cost much
more than common glass, both from the
scarcity of pure pebble and the difficulty
of working so hard material. 1t is,
however, unwise economy, we think, t.
subject so delicate an organ as the eye to
the use of any but the most pure and least
irritating lenses.
FREEZING MIXTURE.
The following directions will, we think,
satisfy the demand of II friend who desires
to know how TO freeze mercury:
Take two or three pounds OF chloride OF
lime, (chlorinated lime) and heat it until
it forms porous mass; powder it and
pass it through seive,b by which operation
it absorbs just enough moisture 10 enable
it to dissolve quickly in water. 1t is then
mixed with half its amount of snow, or
pounded ice, in wooden vessel which is
in mixture ofsnow and salt. If into the
interior of this mixture mercury or ether
be introduced in Il platinum crucible, ii
will soon solidify or freeze.
When this powdered chloride of lime is
dissolved in half or two-thirds its amount
of cold water, it will easily freeze water
when introduced into the mixture in 8
proper vessel.
Other freezing mixtures of much value
are as follows:
|. Sodium carbonate; potassium nitrate
and water, each one part. 2. Ammonium
chloride and sulphate, three parts, ant
water two parts. Either one OF these viii
reduce the temperature about 700 +.
; As these mixtures contain no acids they
are more convenient to handle; and tn
salts they contain may be regained b.
! evaporation of the water and recrystallizin.
the salts.
GREEK FIRE.
J A questions comes to US in this wise
"Whal is Greek fire. Is there any fir
that cannot be put outP" We do no
suppose the last question to have an
theological bearing in this connection, S
: we will try TO answer them both scier
: titically, and both in one. True Greek fir
is simply solid, highly combustibl
composition, very similar, if not identics
" with Carcass composition. What is no'
o commonly called Greek fire consists OF
solution of phosphorus, or sulphur an
phosphorus in II very volatile oil. know
% as carbondisulphide,to which is sometime
added mineral oil, with view of increa
ing its incendiary power.
4 4 When this liquid is thrown on an
Is surface exposed to the air, the so!ven
evaporates rapidly, leaving film
4 ! phosphorus, or phosphorus sulphide, tha
4 inhames spontaneously; the combuslio
g 3 of the phosphorus does not. however
readily set fire to wood or other combust
80 BILE material.
81 1t is not difficult to extinguish this {
Il any other flame by excluding the al
% The proper method in the case undt
rs consideration is to throw on the burnln
q g surface quantity of wet sand, ashe
earth, sawdust, lime or any other powde
wet carpeting or sacking-any materla
,,SS in short, by which the flame can be stltie
by exclusion of the air. No atteml
S should be made to remove the GOVERN
for some time after the flame has bee
N extinguished; and if possible, the plat
g should be thoroughly showered by pow
So erful jet of water.-ProteSS0r broom
% MarJbor, dI,oooJo