AND ON THE ACTION OF LIGHT ON BODIES. 451 
tive one. If the plate be brought into the vapours of mercury 
at the moment when the negative image has disappeared, the 
positive one will be produced, as is the case when the light is 
allowed to continue its action. The images cannot be distin- 
guished from each other, and therefore in this case light and 
mercurial vapours are identical in their effects. 
The most beautiful proof of this proposition is the following :-— 
As far as I am aware the blackened iodide of silver has never 
been prepared except by the influence of light. It cannot be 
obtained by the application of heat, for if an iodized silver plate 
be heated it assumes a milky white appearance on cooling, and 
becomes light-gray when exposed to light; but I will show that 
the blackened iodide may be prepared by means of the vapours 
of mercury. 
It has been already mentioned that these vapours convert a 
correct positive image into a negative one by means of continued 
action, and the plate assumes a yellow appearance the moment 
before, like ordinary iodide of silver. The facts of the case are 
these :—the vapours are first deposited on the bright parts of the 
object (the sky or a landscape) ; on continued action they re- 
move from these parts, and the yellow iodide again makes its 
appearance. If the vapours are allowed to act still longer the 
yellow iodide becomes black, although all other light has been 
excluded and the operation performed in the dark. The nega- 
tive image has been produced. 
I was long since made aware of this curious circumstance,— 
the blackening of iodide of silver by mercurial vapours, inas- 
much as I found it extremely inconvenient in a series of experi- 
ments tending to a different object. It appeared to me possible 
to contrive a simple method of shortening the time necessary for 
the action of the camera obscura, by exposing the iodized plate 
for a moment to the sun, either before or after it has been intro- 
duced into that instrument; but as often as I tried this means, 
and however rapidly I executed it, a blackish colouring was al- 
ways produced when the plate was brought into the mercurial 
vapours, although it had exhibited its usual yellow colour be- 
fore being exposed to them. I must also mention, that the black 
iodide produced by mercury adheres to the plate just as little, 
and is not more easily soluble in hyposulphite of soda than that 
formed by light. 
Although the identity of the action of light and mercurial va- 
