of the Solar Spectrum on Vegetable Colours. 249 
plausible notion of the chemical nature of that peculiar inter- 
mediate oxide to which the name of “ Ferroso-ferric” has been 
given by Berzelius. If (to render my meaning more clear) we 
for a moment consent to designate such an isomeric form of 
iron by the name siderium, the oxide in question might be 
regarded asa sideriate of iron. Both 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 sul- 
phocyanate 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 discoloration begins to relax, and the paper when dry re- 
tains 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, ap- 
pears at the back of the paper with even more distinctness than 
on its face. ‘The impression, however, is pallid ; fades on keep- 
ing, nor am | 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 proportions, 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 so- 
lution of proto-nitrate of mercury, which in a little time en- 
tirely 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 ex- 
posing to a very concentrated spectrum (collected by a lens 
* The latter from the late experiments and remarks of Rose on the vi- 
treous state of the arsenious acid and its luminosity in crystallizing from 
acid solutions. [See Phil. Mag., S. 3. vol. vii. p. 534,—Eprr. ] 
