f 
OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
213 
225. Whatever be the state of the iron in the double salts in question, its reduc- 
tion by blue light to the state of protoxide is indicated by many other reagents. If, 
for example, a slip of paper, prepared with the ammonio-citrate and partially sunned, 
be washed, when withdrawn, with bichromate of potash, the bichromate is deoxidized 
and precipitated on the sunned portion, just as it would be if directly exposed to the 
sun’s rays. Every reagent in short which is susceptible of being deoxidated, wholly or 
in part, by contact with the protoxide of iron, is so also by contact with the sunned 
paper. Taking advantage of this property, I have been enabled to add another and 
very powerful element to the list of photographic ingredients. 
226. Photographic Properties of Mercury . — This element is mercury. As an agent 
in the Daguerreotype process, it is not, strictly speaking, photographically affected. It 
operates there only in virtue of its readiness to amalgamate with silver, properly pre- 
pared to receive it. That it possesses direct photographic susceptibility, however, in 
a very eminent degree, is proved by the following experiment. Let a paper be washed 
over with a weak solution of periodide of iron, and when dry with a solution of proto 
nitrate of mercury. A bright yellow paper is produced, which (if the right strength 
of the liquids be hit) is exceedingly sensitive while wet, darkening to a brown colour in 
a very few seconds in the sunshine. Withdrawn, the impression fades rapidly, and the 
paper in a few hours recovers its original colour. In operating this change of colour 
the whole spectrum is effective, with the exception of the thermic rays beyond the 
red. 
227- Proto-nitrate of mercury simply washed over paper is slowly and feebly black- 
ened by exposure to sunshine. And if paper be impregnated with the ammonio-ci- 
trate, already so often mentioned, partially sunned, and then washed with the proto- 
nitrate, a reduction of the latter salt, and consequently blackening of the paper takes 
place very slowly in the dark over the sunned portion, to nearly the same amount as 
in the direct action of the light on the simply nitrated paper. 
228. But if the mercurial salt be subjected to the action of light in contact with 
the ammonio-citrate, or tartrate, the effect is far more powerful. Considering, at pre- 
sent, only the citric double-salt, a paper prepared by washing first with that salt and 
then with the mercurial proto-nitrate (drying between) is endowed with considerable 
sensibility, and darkens to a very deep brown, nay to complete blackness, on a mo- 
derate exposure to good sun. Very sharp and intense photographs of a negative cha- 
racter may be thus taken. They are however difficult to fix. The only method 
which I have found at all to succeed, has been by washing them with bichromate of 
potash and soaking them for twenty-four hours in water, which dissolves out the 
chromate of mercury for the most part, leaving however a yellow tint on the ground, 
which resists obstinately. But though pretty effectually fixed in this way against 
light , they are not so against time, as they fade considerably on keeping. 
229. When the proto-nitrate of mercury is mixed, in solution, with either of the 
atnmoniacal double salts, it forms a precipitate, which, worked up with a brush to the 
