250 Sir J. F. W. Herschel on the Action of the Rays 
of short focus) a slip of paper duly prepared as above (that is 
to say, by washing with the mixed solutions, exposure to sun- 
shine, washing, and discharging the uniform blue colour so 
induced as in the last article), its whiteness is changed to 
brown over the whole region of the red and orange rays, bed 
not beyond the luminous spectrum, ‘Three conclusions seem 
unavoidable ;—1st, that it is the heat of these rays, not their 
light, which operates the change; 2ndly, that this heat pos- 
sesses a peculiar chemical quality which is not possessed by 
the purely calorific rays outside of the visible spectrum, though 
far more intense; and, 3rdly, that the heat radiated from ob- 
scurely hot iron, abounds especially in rays analogous to those 
of the region of the spectrum above indicated. And there are 
the very same conclusions derived from the experiments on 
guaiacum in Art. 158—160, 
225. Whatever be the state of the iron in the double salts 
in question, its reduction 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sunned, be washed, when withdrawn, with bichromate of 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 ingre- 
dients. 
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 prepared to receive it. That it possesses direct pho- 
tographic 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 Withdrawn, 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 co- 
lour 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 blackened by exposure to sunshine, And 
