12 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
greater part of the liquid was decanted off with a siphon tube, and the last portions 
very slowly and cautiously drained away, drop by drop, by a siphon composed of a 
few fibres of hemp, laid parallel and moistened without twisting. The glass was not 
moved till quite dry, and was found coated with a pretty uniform film of chloride of 
silver, of delicate tenuity and chemical purity, which adhered with considerable force, 
and was very little sensible to light. On dropping on it a solution of nitrate of silver, 
however, and spreading it over by inclining the plate to and fro, (which it bore with- 
out disturbing the film of chloride,) it became highly sensitive, although no organic 
matter could have been introduced with the nitrate, which was quite pure, nor could 
any indeed have been present, unless it be supposed to have emanated from the 
hempen filaments which were barely in contact with the edge of the glass, and which 
were constantly abstracting matter from its surface in place of introducing new. 
36. Exposed in this state to the focus of a camera, with the glass towards the in- 
cident light, it became impressed with a remarkably well-defined negative picture, 
which was direct or reversed according as looked at from the front or the back. On 
pouring over this, cautiously by means of a pipette, a solution of hyposulphite of 
soda, the picture disappeared, but this was only while wet, for on washing in pure 
water and drying, it was restored, and assumed much the air of a Daguerrotype 
when laid on a black ground, and still more so when smoked at the back, the silvered 
portions reflecting most light, so that its character had, in fact, changed from negative 
to positive. From such a picture (of course before smoking) I have found it practi- 
cable to take photographic copies ; and although I did not, in fact, succeed in attempt- 
ing to thicken the film of silver, by connecting it, under a weak solution of that metal, 
with the reducing pole of a voltaic pile, the attempt afforded distinct indications of 
its practicability with patience and perseverance, as here and there, over some small 
portions of the surface, the lights had assumed a full metallic brilliancy under this pro- 
cess. I would only mention further to those who may think this experiment worth 
repeating, that all my attempts to secure a good result by drying the nitrate on the 
film of chloride have failed, the crystallization of the salt disturbing the uniformity 
of the coating. To obtain delicate pictures, the plate must be exposed wet, and when 
withdrawn must immediately be plunged into water. The nitrate being thus abs- 
tracted, the plate may then be dried, in which state it is half-fixed, and is then ready 
for the hyposulphite. Such details of manipulation may appear minute, but they 
cannot be dispensed with in practice, and cost a great deal of time and trouble to 
discover. 
37. This mode of coating glass with films of precipitated argentine or other com- 
pounds, affords, it may be observed, the only effectual means of studying their habi- 
tudes on exposure to light, free from the powerful and ever-varying influence of the 
size in paper, and other materials used in its manufacture, and estimating their de- 
gree of sensibility and other particulars of their deportment under the influence of 
reagents. I find, for example, that glass so coated with the iodide of silver is much 
