248 Sir J. F. W. Herschel on the Action of the Rays 
to the prussiated solution, by which it is hindered from spread- 
ing unmanageably within the pores of the paper, and the pre- 
cipitated Prussian blue allowed time to agglomerate and fix 
itself on the fibres. By the use of this ingredient also, a much 
thinner and more equable film may be spread over the sur- 
face ; and when perfectly dry, if not sufficiently developed, the 
application may be repeated. By operating thus I have oc- 
casionally (though rarely) succeeded in producing pictures of 
great beauty and richness of effect, which they retain (if not 
thrown into water) between the leaves of a portfolio, and have 
even a certain degree of fixity—fading in a strong light and 
recovering their tone in the dark. The manipulations of this 
process are, however, delicate, and complete success is com- 
paratively rare. 
221. If sulphocyanate of potash be added to the ammonio- 
citrate or ammonio-tartrate of iron, the peculiar red colour 
which that test induces on persalts of the metal is not pro- 
duced, but appears at once on adding a drop or two of dilute 
sulphuric or nitric acid. ‘This circumstance, joined to the 
perfect neutrality of these salts, and their power, in such 
neutral solution, of enduring, undecomposed, a boiling heat, 
contrary to the usual habitudes of the peroxide of iron *, 
together with their singular transformation by the action of 
light to proto-salts, in apparent opposition to a very strong . 
affinity, has, I confess, inclined me to speculate on the possi- 
bility of their ferruginous base existing in them, not in the or- 
dinary form of peroxide, but in one isomeric with it. The 
non-formation of Prussian blue, when their solutions are mixed 
with prussiate of potash (Art. 209), and the formation in its 
place of a deep violet-coloured liquid of singular instability 
under the action of light, seems to favour this idea. Nor is 
it altogether impossible that the peculiar “ prepared” state 
superficially assumed by iron under the influence of nitric 
acid, first noticed by Keir, and since made the subject of ex- 
periment by M. Schénbein and myself}, may depend on a 
change superficially operated on the zron itself into a new me- 
tallic body isomeric with iron, unoxidable by nitric acid, and 
which may be considered as the radical of that peroxide 
which exists in the salts in question, and possibly also of an 
isomeric protoxide. A combination of the common protoxide 
with the isomeric peroxide, rather than with the same metal 
in a simply higher stage of oxidation, would afford a not un- 
* See my paper on this subject in Philosophical Transactions, cxi. p. 293, 
{or Phil. Mag., S. 1. vol. lix. p. 86.—Epir. }. 
+ See Annales de Chimie, tom. liv. p. 87, [or Phil. Mag., S. 3. vol. xi. 
p. 329.—Enir. ]. 
