1864. | Photography. 351 
the action may be judged by measurement with the galvanometer, which 
will be found in a cell where the copper and zine surfaces are about 3 
inches square each, to have its needle deflected to about 20°. This is 
in the case of the meter ordinarily employed by telegraphists. The 
power may be increased by approaching the elements of the cells nearer 
to one another, by placing the copper over the sulphate, and by reduc- 
ing the beads to half an inch. There is, however, danger in this case 
of the accumulating surface of copper on the copper plate, crystallizing 
in an upward direction, and ultimately shooting out a fibre which may 
touch the zinc plate above it, when of course the cell would cease to 
work. The first arrangement is the best, and it is the one which 
M. Bonelli has adopted in working his new telegraph. The above 
battery appears to be the simplest yet brought before the public, con- 
sisting as it does of easily procurable materials, and its action being 
only limited by the waste of zinc and evaporation of water. M. Bonelli 
uses 30 cells to work his five-wire telegraph for a long distance, so 
that 6 cells appear amply sufficient per wire. 
IX. PHOTOGRAPHY. 
TopipE of silver may be looked upon as the foundation of the present 
photographie art, and yet up to a very recent period, men of science 
have differed in their opinion as to whether this compound is photo- 
graphically sensitive per se or not. As late as January last, we find 
a foreign photograher, M. Gaudin,* publishing some experiments which 
tend to show that iodide of silver, when prepared by the direct com- 
bination of iodine and silver-leaf, is absolutely insensitive to light 
when tested under a negative, and that an hour’s exposure in the 
printing frame does not give rise to any impression whatever, either 
before or after the application of a developing agent. The iodide of 
silver used in these experiments was in the form of a yellow powder, 
rather unctuous to the touch, and was merely rubbed over the surface 
of a piece of paper with a tuft of cotton. Subsequent experiments 
were tried in which the iodide of silver, after being rubbed over the 
paper, was exposed to the vapours of nitric acid, hydrochloric acid, 
and ammonia: the last only gave signs of a picture, and that only 
after a very long exposure. These’ experiments tend to confirm what 
is generally stated in chemical works; but Mr. Spiller ¢ has since 
communicated some further results, tried with that care and ingenuity 
which are so well known to those who are familiar with the published 
researches of this chemist, which show conclusively that iodide of 
silver exists in two different modifications, one of which is insensitive, 
whilst the other is sensitive to the action of light. Reasoning upon 
the well-known change which takes place when iodide of mercury is 
heated—its scarlet colour changing to yellow—it occurred to him that 
a similar change might possibly be induced in the iodide of silver. This 
* «The Photographie News,’ vol. viii. p. 8. + Ibid. vol. viii. p. 15. 
