of the Solar Spectrum on Vegetable Colours. 251 
if paper be impregnated with the ammonio-citrate, already 
so often mentioned, partially sunned, and then washed with 
the proto-nitrate, a reduction of the latter salt, and conse- 
quently 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 present, only the 
citric double-salt, a paper prepared by washing first with that 
salt and then with the mercurial proto-nitrate (drying be- 
tween) is endowed with considerable sensibility, and darkness 
to a very deep brown, nay to complete blackness, on amo- 
derate exposure to good sun. Very sharp and intense photo- 
graphs of a negative character inay be thus taken. They are 
however difficult to fiz. 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 consi- 
derably on keeping. 
229. When the proto-nitrate of mercury is mixed, in solu- 
tion, with either of the ammoniacal double salts, it forms a 
precipitate, which, worked up with a brush to the consistency 
of cream, is easily (and with certain precautions of manipula- 
tion*) very evenly spread on paper, producing photographic 
tablets of every variety of sensibility and inertness, accordin 
to the proportion of the doses used. By combining all three 
of the ingredients, and adding a small quantity of tartaric 
acid +, a paper is produced of a pretty high degree of sensi- 
bility (more than by the use of either separately), which in 
about half an hour or an hour, according to the sun, affords 
pictures of such force and depth of colour, such velvety rich- 
ness of material, and such perfection of detail and preserva- 
tion of the relative intensities of the light, as infinitely to sur- 
* The cream should be spread as rapidly as possible over the whole 
paper, well worked in, cleared off as much as possible, and finished with a 
brush nearly dry, spread out broad and pressed toa straight thin edge, which 
must be drawn as lightly and evenly as possible over every part of the paper 
till the surface appears free from every streak, and barely moist. 
t+ One measure of a solution of ammonio-citrate, and one of a solution 
of ammonio-tartrate of iron, containing, each, one-tenth of its weight of 
the respective salts, Tartaric acid, saturated solution, one-eighth of the 
Joint volumes of the other solutions. Form a cream by pouring in as rapidly 
as possible one measure of a saturated solution of the proto-nitrate and 
well mixing with a brush. 
