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change in the glass ; I mean the action of water or of aerial
moisture, which is frequently considerable, and appears to be
dependent upon the alkali present.

1 10. If a small quantity of flint glass be very finely pulverized



282 On the Manufacture of Optical Glass. [1829.

in an agate mortar, then placed upon a piece of turmeric paper,
and moistened with a drop of pure water, strong indications of
free alkali will be obtained. The same effect is produced by
using plate glass; and if the pulverization be very perfect, the
alkali can be detected in glasses containing far smaller quantities
of that substance than either of those mentioned. This experi-
ment, due to Mr. Griffiths, shows that in whatever state of
combination the alkali may be, it can still act upon, and is
subject to, the action of moisture ; and that flint glass is by no
means a compound resulting from very strong chemical affinities,
is also shown by an experiment which I made many years ago ;
namely, that if flint glass be pulverized exceedingly fine, the
powder will indicate the presence of sulphuretted hydrogen in
the air by becoming blackened, almost as readily as carbonate of
lead. Glass may be considered rather as a solution of different
substances one in another, than as a strong chemical compound ;
and it owes its power of resisting agents generally to its perfectly
compact state, and the existence of an insoluble and unchange-
able film of silica, or highly silicated matter upon its surface.

111. The half-combined and hygrometric state of the alkali
appears to be the cause of the deposited film of moisture which
is well known to adhere to ordinary glass when exposed to the
atmosphere at common temperatures. This film is highly
calculated to condense any portion of sulphuretted vapours
which may be floating in the atmosphere, and thus bring them
into contact with the oxide of lead under the most favourable
conditions for the production of that action which is the direct
cause of tarnish. Now from this cause of action the heavy
glass is free ; and hence a satisfactory reason to me why the
heavy glasses have suffered so little when left with common
care in an ordinary atmosphere.

1 12. An extraordinary difference exists between the electrical
relations of this glass and other glasses, due principally to the
same absence of alkali. Ordinary glasses, either flint, plate or
crown, will, from the hygrometric film of moisture upon the
surface, freely conduct electricity tinder common circumstances.
Thus if a gold-leaf electrometer be diverged, and then touched
with them in their ordinary state, the electricity is instantly
discharged, even though the hand be two or three feet from
the part touching the instrument. If a similar experiment be



1829.] On the Manufacture of Optical Glass. 283

made with these heavy glasses, they have no sensible power of
discharging the electricity, but insulate as perfectly as sealing-
wax or gum-lac. If one of these plates of glass, without any
previous warming and drying, be lightly brushed or wiped with
flannel or silk, it instantly becomes strongly electrical, and
retains its electricity for a long time ; but it would be almost
impossible to develope electricity by such slight means with
flint or plate or even crown glass in a similar state. Hence
the glass makes as good an electrophorus as lac or resin, and
may probably be found hereafter to answer many useful elec-
trical purposes. But the great point at present in view, is the
proof which such electrical properties give of the absence of
that film of moisture which is so constant upon other glasses.

113. All these circumstances are favourable to the opinion
that the heavy glass will not be found objectionable in the con-
struction of telescopes, because of any undue tendency to
tarnish, and especially when precautions are taken to protect
it from sulphuretted vapours in the manner before described
(107). No difficulty can be anticipated in preserving the air
within a limited and enclosed space free from such conta-
mination : to preserve it dry, if that had been necessary, under
the different circumstances of varying temperature and the
inevitable change of the air more or less frequently, would have
been a far more difficult task.

114. The other kind of superficial change, i. e. the corrosion
or crystallization which takes place principally on plate glass,
is doubtless also due to the alkali present. Sometimes, indeed,
specimens of glass may be found where the alkali being too
abundant, a similar but more extensive action has taken place
over the whole of the surface, and the glass falls off in scales.
Whether the alteration be due to the action of the alkali on
the water only, or on the carbonic acid and other substances it
finds in the air, or to its united action on all together, is of little
consequence at present, as the substance on which it depends
is altogether absent from the glass under consideration.

115. Among the great number of glasses made, there are
several of different composition, which have been, selected,
because of their general characters and properties, for more
extensive trial and investigation when time will permit. Of
these it would be useless to speak at present, as what might be



284 On the Manufacture of Optical Glass. [1829.

stated of them now would probably require correction from
future experiments. Up to this period the attention has been
devoted, as it still must be for a while, to the establishment of
a process which, competent to produce with certainty a glass
fitted for optical purposes, may have the philosophy and practice
of every part so fully ascertained, as to be capable of description
in a manner sufficiently clear to enable any other person, with
moderate care, to obtain the same results without the labour of
long and tedious investigation.



APPENDIX.

Rough- glass furnace. The only furnace for making rough
glass which has been constructed, answers its purpose exceed-
ingly well ; and though if a second were to be made, it should
be upon a larger scale, yet I think it better to describe the tried
one accurately, than to direct alterations which have not been
experimentally approved of; especially as there seems to be
nothing which, in principle, need differ in a larger furnace.
An iron box (Plate II.) 30 inches long, 14 inches wide, and
8J inches deep, forms the principal part of the exterior : it is
open entirely at the top, and at the bottom also, in the fore
part, where a fire-grate is to be placed. It has a common iron
furnace door in front, the aperture of which is 8 inches wide
by 6 inches high ; and at the opposite end, or back of the
furnace, a flanched aperture, 6| inches by 4| for a piece of
funnel pipe to connect the furnace with a powerful flue. The
sides of this box, and such part of the bottom as is not appro-
priated for the fire-grate, are lined with fire-stone 1^ inch in
thickness, except in the fire-place, where it is 2f . The grate
is 12 inches long by 8 wide ; and the part above it is closed
by a fire-tile 2 inches thick and 12 inches square, which resting
on the edges of the lining, finishes the portion intended for the
coal fire, leaving it 5 inches in depth from the covering tile to
the grate. The other part is covered by an iron plate, 17-
inches long, 13 inches wide, and |ths of an inch thick, which,
resting upon the edges of the lining, encloses a space of 16 inches
long, 10 inches wide, and 5 inches deep, for the reception of



1S29.] On the Manufacture of Optical Glass. 285

crucibles. This plate is formed with circular holes, about 3
inches, or rather more, in diameter, arranged as in fig. 1, that
the crucibles inserted through them may leave plenty of room
for the intervention of coke and flame. As many round crucible-
covers belong to the plate as there are holes, serving to close
such of them as are not occupied by crucibles.

As the plate becomes very hot when in use, it is necessary
to have a second above it, which may be formed of sheet-iron
with corresponding holes, and when put into its place, separated
from the first, a little space, by pieces of tobacco pipe, or other
convenient substance, to include a layer of air. But it is much
better for the retention of heat, and also for its superior cleanli-
ness, that this second plate should consist of pieces of earthen-
ware fitted to each other, so as to cover the surface of the iron
plate, from which it should also be separated by a short interval.

The crucibles used are 5 inches high outside, 3^ inches dia-
meter at the top, and 2 inches diameter at the bottom. They
are of pure porcelain biscuit, perfectly white and clean. They
should be made as thin as possible, of the finest and most
refractory kind of ware, and baked at a high temperature.
We have some crucibles made about thirty years ago for
Mr. Hatchett, which, though not of the size required, are pre-
cisely the right kind of ware. They have been used many times
in succession without cracking or being importantly acted upon
by the glass, and no sensible degree of impurity was given to
it from them.

When these crucibles are arranged in the furnace, they
should be supported by little stands of earthenware, formed out
of brick or Cornish tile, so that their edges shall rise about
,T or yi'd of an inch above the surface of the upper covering plate,
that no impurity may enter them. The holes in the plate should
be of such dimensions that, when hot, the crucibles may fit
loosely, that they may be uninjured, and also that there may be
room between for the vapours that are evolved from the mixture
to pass away.

The covers to the crucibles are evaporating basins about 4^
inches in diameter. They are slung with their edges downwards
by pieces of platinum wire sufficiently strong for the purpose,
which being first bent at the middle into an angle, are then
stretched across the outside of the basins, and have their ends



286 On the Manufacture of Optical Glass. [1S2&

bent round the opposite edges of the latter. The bent extremity
of an iron rod passed under the loop thus formed over the middle
of the bottom, serves to raise and remove any cover from place
to place. When a crucible is in use, the cover should be
arranged over it in such a manner as not to touch the vessel,
but rest by its ecjges on the earthenware plate around.

The platinum stirrers in use with this furnace have been before
described (28. 7.5), fig. 3. The platinum ladle consists of a small
crucible of that metal riveted to a platinum wire, and that made
fast by a screw to an iron rod (fig. 4).

The use and manner of working this furnace will be well
understood from the above description, and what has before
been said (26, &c.). The crucible should never be suddenly
heated or cooled. The coke may be fed and arranged at such
of the crucible holes as are out of use at the time. Because of
the very valuable effects of a trough of water under the fire-bars
(45) experienced in the larger furnace, one is constantly used
with that just described.

Finishing furnace. This furnace on the outside is a paral-
lelepiped, principally of brickwork, built against a wall ; it is 64
inches in length, from the fire front to the beginning of the flue,
against which it is built, 45 inches wide, and 28 inches high
(figs. 5, 6 & 7). It is the only one that has yet been built, and,
for the reasons before given, shall be described exactly as it is.
The fire-place is at one end, and the course of the flame and
smoke is directly from that to the other end, and then imme-
diately into the upright flue. The fire-place is 15 inches from
back to front, 13 inches wide, and 11 \ inches from the arched
roof to the bars. Its outward side, or that from the wall, is
18J inches in thickness of brickwork, which is intended to give
stability to the structure. The mouth of the fire-place is an
aperture 8 inches by 6 inches, made in a piece of fire- stone
7 inches inwards from the front of the brickwork : its lower
edge is level with a fire- stone sill, which, extending forwards
from the fire-place to the outer surface of the brickwork, forms
a shelf, on which two bricks stand, that serve in place of a door
to close the mouth of the furnace. The ash-pit is 25 inches
long, 12 inches wide under the fire, and 10 inches high to thq
bars. A trough made of rolled iron, riveted together, and 5~
inches high on the sides, occupies its lower part. This being



1829.] On the Manufacture of Optical Glass. 287

preserved full of water, is sustained at the boiling temperature
by the radiation of heat and the hot ashes which fall into it.

From the back part of the fire-place, and 2 inches above the
level of the fire-bars, the brickwork is carried on horizontally
until close to the stack. The sides of this part are perpen-
dicular, and 12 inches apart: they are continued upwards to
the top of the brickwork 14 inches unbroken, except that at
5 inches from the bottom they are thrown back Jrd of an inch
so as to form a ledge there. This ledge is for the purpose of
receiving the edges of certain fire-tiles, which, when put in,
form the top of the flue and at the same time the bottom of
the glass chamber ; but the whole is so constructed, that the
tiles can be put in and taken out at pleasure without disturbing
the rest of the work. The side, or rather end of the chamber
nearest the fire, is constructed of a fire-tile, which terminates
and faces the brick arch over the fire-place, and extends from
the surface of the brickwork downwards 9 inches to the side
ledges before described: the further end of the chamber is
finished in a similar way, and beyond that the flue is carried in
the most convenient and direct manner, but without any unne-
cessary contraction, into the stack or chimney. The length of
this upper aperture, afterwards constituting the chamber, is
25 inches, its breadth 12 inches. When the bottom tiles are
in their places, they leave a depth of 5 inches for that part of
the furnace or flue beneath the chamber, which is also 38 inches
from the fire to the end, and, with the exception of certain
supports in it, is 12 inches wide.

These supports are built in with the bottom of the flue.
They are essential to the permanency and regularity of the
bottom of the glass chamber, and require considerable nicety
in their arrangement. They consist of fire-bricks placed up on
end, so that their narrowest surfaces are towards the ends of
the furnace, their sides or broadest exposed surfaces parallel
with the sides of the furnace itself. They rise to the same
height above the bottom of the flue as the ledges on the sides
of the brickwork, or 5 inches ; and with them form the support
for the bottom tiles. There are three of them in the furnace,
placed in a line equidistant from the two sides of the flue ; and
being 2~ inches thick, they leave spaces for the passage of flame
and the reception of coke, which are 4f inches in width. The



290 On tJie Manufacture of Optical Glass, [1829.

a slag and coke rake (89) ; and a stoking iron, with its extre-
mity bent, for the purpose of breaking the clinkers off the bars
from beneath upwards.

Preparation of spongy platinum. The platinum used for
this preparation should be pure, and may be the refuse pieces
resulting from such plate and foil as has been in use for trays
in former experiments. This, after being taken out of the
pickle (93), and condemned as useless for other purposes in
the glass house, should be trimmed from all alloyed parts, if
any such are adhering to it, and then digested in a Florence
flask, with a mixture of five measures of strong hydrochloric acid,
one measure of strong nitric acid, and three measures of water.
But little heat should be applied at first until the action di-
minishes. According to Dr. Wollaston, one ounce of platinum
will be dissolved by about four ounce measures of such acid,
and it is advantageous to have a considerable excess of pla-
tinum present. The solution obtained is to be precipitated by a
strong solution of muriate of ammonia ; a bright yellow pul-
verulent substance will fall, and a mother-liquid having more
or less colour remain. The precipitate being allowed to sub-
side, the liquor is to be poured off, and the former then washed
with two or three portions of water. The washing liquors and
the mother-water may afterwards be concentrated together;
but it is better not to prepare spongy platinum for this particular
use from these fluids, but only from the precipitate which falls
on adding the muriate of ammonia.

The yellow precipitate, when washed, is to be dried on a
filter, or in a basin, and then decomposed by the application of
a dull red heat. This may be done in a clean white earthenware
crucible. The heat should be continued until vapours cease to
arise ; but this will be found a long operation, in consequence of
the low temperature which is to be applied, and the exceedingly
bad conducting power of platinum for heat when in this spongy
state. The reduction may also be performed by putting the pre-
cipitate upon a piece of platinum foil in a layer about th of an
inch in thickness, and covering it with another piece of foil ; a
spirit lamp will then suffice to reduce the metal, but the foil
and powder must be turned occasionally, that both sides may be
exposed to the flame. The platinum will appear as a dull grey
spongy metallic mass. It should be broken up, mingled, and



1831.] On a Peculiar Class of Optical Deceptions. 291

then again heated to ensure the dissipation of all volatile
matter.

After this is done, the platinum should be rubbed to powder
by the clean finger on clean paper (83), heated slightly a third
time, and then preserved in a clean and well-stopped bottle.



On a Peculiar Class of Optical Deceptions *.

THE pre-eminent importance of the eye as an organ of per-
ception confers an interest upon the various modes in which
it performs its office, the circumstances which modify its indi-
cations, and the deceptions to which it is liable, far beyond
what they otherwise would possess. The following account of
a peculiar ocular deception, which, in a greater or smaller
degree, is not uncommon, and which, if looked for, may be
observed with the utmost facility, may therefore prove worthy
of attention ; and I am the more inclined to hope so, because
in some points it associates with an account and explanation of
an ocular deception given by Dr. Roget in the f Philosophical
Transactions' for 1825, page 121.

The following are some cases of the appearance in question.
Being at the magnificent lead mills of Messrs. Maltby, two
cog-wheels were shown me moving with such velocity, that if
the eye were retained immoveable, no distinct appearance of
the cogs in either could be observed ; but, upon standing in
such a position that one wheel appeared behind the other,
there was immediately the distinct though shadowy resem-
blance of cogs moving slowly in one direction.

Mr. Brunei, jun. described to me two small similar wheels
at the Thames Tunnel: an endless rope, which passed over
and was carried by one of them, immediately returned and
passed in the opposite direction over the other, and conse-
quently moved the two wheels in opposite directions with great

* Quarterly Journal of Science, 1831, vol. i. p. 205.

I take the opportunity here of pointing out that, three years prior to my
paper, Professor Plateau had published an account of the chief fact, in the
fourth volume of the ' Correspondence Mathematique et Physique ' of M.
Quetelet, p. 393. I was of course unaware of the circumstance. Further
observations by M. Plateau will be found in the 'Annales de Chimie,' 1831,
xlviii. p. 281.

V2



2DO On tfo Manufacture of Optical Glass. [1829.

a slag and coke rake (89) ; and a stoking iron, with its extre-
mity bent, for the purpose of breaking the clinkers off the bars
from beneath upwardsr

Preparation of spongy platinum. The platinum used for
this preparation should be pure, and may be the refuse pieces
resulting from such plate and foil as has been in use for trays
in former experiments. This, after being taken out of the
pickle (93), and condemned as useless for other purposes in
the glass house, should be trimmed from all alloyed parts, if
any such are adhering to it, and then digested in a Florence
flask, with a mixture of five measures of strong hydrochloric acid,
one measure of strong nitric acid, and three measures of water.
But little heat should be applied at first until the action di-
minishes. According to Dr. Wollaston, one ounce of platinum
will be dissolved by about four ounce measures of such acid,
and it is advantageous to have a considerable excess of pla-
tinum present. The solution obtained is to be precipitated by a
strong solution of muriate of ammonia ; a bright yellow pul-
verulent substance will fall, and a mother-liquid having more
or less colour remain. The precipitate being allowed to sub-
side, the liquor is to be poured off, and the former then washed
with two or three portions of water. The washing liquors and
the mother-water may afterwards be concentrated together;
but it is better not to prepare spongy platinum for this particular
use from these fluids, but only from the precipitate which falls
on adding the muriate of ammonia.

The yellow precipitate, when washed, is to be dried on a
filter, or in a basin, and then decomposed by the application of
a dull red heat. This may be done in a clean white earthenware
crucible. The heat should be continued until vapours cease to
arise ; but this will be found a long operation, in consequence of
the low temperature which is to be applied, and the exceedingly
bad conducting power of platinum for heat when in this spongy
state. The reduction may also be performed by putting the pre-
cipitate upon a piece of platinum foil in a layer about th of an
inch in thickness, and covering it with another piece of foil ; a
spirit lamp will then suffice to reduce the metal, but the foil
and powder must be turned occasionally, that both sides may be
exposed to the flame. The platinum will appear as a dull grey
spongy metallic mass. It should be broken up, mingled, and



1831.] On a Peculiar Class of Optical Deceptions. 291

then again heated to ensure the dissipation of all volatile
matter.

After this is done, the platinum should be rubbed to powder
by the clean finger on clean paper (83), heated slightly a third
time, and then preserved in a clean and well-stopped bottle.



On a Peculiar Class of Optical Deceptions *.

THE pre-eminent importance of the eye as an organ of per-
ception confers an interest upon the various modes in which
it performs its office, the circumstances which modify its indi-
cations, and the deceptions to which it is liable, far beyond
what they otherwise would possess. The following account of
a peculiar ocular deception, which, in a greater or smaller
degree, is not uncommon, and which, if looked for, may be
observed with the utmost facility, may therefore prove worthy
of attention ; and I am the more inclined to hope so, because
in some points it associates with an account and explanation of
an ocular deception given by Dr. Roget in the ' Philosophical
Transactions' for 1825, page 121.

The following are some cases of the appearance in question.
Being at the magnificent lead mills of Messrs. Maltby, two
cog-wheels were shown me moving with such velocity, that if
the eye were retained immoveable, no distinct appearance of
the cogs in either could be observed ; but, upon standing in
such a position that one wheel appeared behind the other,
there was immediately the distinct though shadowy resem-
blance of cogs moving slowly in one direction.

Mr. Brunei, jun. described to me two small similar wheels
at the Thames Tunnel: an endless rope, which passed over
and was carried by one of them, immediately returned and
passed in the opposite direction over the other, and conse-
quently moved the two wheels in opposite directions with great

* Quarterly Journal of Science, 1831, vol. i. p. 205.

I take the opportunity here of pointing out that, three years prior to my
paper, Professor Plateau had published an account of the chief fact, in the
fourth volume of the ' Correspondence Mathematique et Physique ' of M.
Quetelet, p. 393. I was of course unaware of the circumstance. Further
observations by M. Plateau will be found in the ' Annales de Chimie/ 1831,
xlviii. p. 281.

u2



OH a Peculiar Class of Optical Deceptions. [1831.

but equal velocities. When looked at from a particular posi-



Online LibraryMichael FaradayExperimental researches in chemistry and physics → online text (page 28 of 49)