212 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
phosphorus and arsenic (bodies remarkable for sesqui-combinations) admit isomeric 
forms in their oxides and acids*. But to return from this digression. 
222. If to a mixture of ammonio-citrate of iron and sulphocyanate of potash a small 
dose of nitric acid be added, the resulting red liquid spread on paper spontaneously 
whitens in the dark. If more acid be added till the point is attained when the dis- 
coloration begins to relax, and the paper when dry retains a considerable degree of 
colour, it is powerfully affected by light, and receives a positive picture with great 
rapidity, which, like the guaiacum impression noticed in Art. 154, appears at the 
back of the paper with even more distinctness than on its face. The impression, 
however, is pallid ; fades on keeping, nor am I acquainted at present with any mode 
of fixing it. 
223. If paper be washed with a mixture of the solutions of ammonio-citrate of iron 
and ferrosesquicyanate of potash, so as to contain the two salts in about equal pro- 
portions, and being then impressed with a picture, be thrown into water and dried, a 
negative blue picture will be produced agreeably to what is stated in Art. 154. This 
picture I have found to be susceptible of a very curious transformation, preceded by 
total obliteration. To effect this it must be washed with solution of proto-nitrate of 
mercury, which in a little time entirely discharges it. The nitrate being thoroughly 
washed out and the picture dried, a smooth iron is to be passed over it, somewhat 
hotter than is used for ironing linen, but not sufficiently so to scorch or injure the 
paper. The obliterated picture immediately reappears, not blue, but brown. If kept 
for some weeks in this state between the leaves of a portfolio, in complete darkness, 
it fades, and at length almost entirely disappears. But what is very singular, a fresh 
application of the heat revives and restores it to its full intensity. 
224. This curious transformation is instructive in another way. It is not operated 
by light, at least not by light alone. A certain temperature must be attained, and 
that temperature suffices in total darkness. Nevertheless, I find that on exposing to 
a very concentrated spectrum (collected by a lens of short focus) a slip of paper duly 
prepared as above (that is to say, by washing with the mixed solutions, exposure to 
sunshine, 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, but not beyond the luminous spectrum. Three conclusions seem un- 
avoidable ; — 1st, that it is the heat of these rays, not their light, which operates the 
change ; 2ndly, that this heat possesses 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 obscurely hot iron, abounds espe- 
cially 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. 
* The latter from the late experiments and remarks of Rose on the vitreous state of the arsenious acid and 
its luminosity in crystallizing from acid solutions. 
