Action of the Solar Spectrum on Vegetable Colours. 247 
picture may be fixed by the hyposulphite of soda, which alone, 
I believe, can be fully depended on for fixing argentine pho- 
tographs. 
219. Cyanotype.—If a nomenclature of this kind be admitted 
(and it has some recommendations), the whole class of pro- 
cesses in which cyanogen in its combinations with iron per- 
forms a leading part, and in which the resulting pictures are 
blue, may be designated by this epithet. The varieties of cy- 
anotype processes seem to be innumerable, but that which 
I shall now describe deserves particular notice, not only for 
its pre-eminent beauty while in progress, but as illustrating 
the peculiar power of the ammoniacal and other persalts of 
iron above mentioned to receive a latent picture, susceptible 
of development by a great variety of stimuli. This process 
consists in simply passing over the ammonio-citrated paper 
on which such a latent picture has been impressed, very 
sparingly and evenly, a wash of the solution of the common 
yellow ferrocyanate (prussiate) of potash*. The latent pic- 
ture, if not so faint as to be quite invisible (and for this pur- 
pese it should not be so), is negative. As soon as the liquid 
is applied, which cannot be in too thin a film, the negative 
picture vanishes, and by very slow degrees is replaced by a 
positive one of a violet-blue colour on a greenish yellow 
ground, which at a certain moment possesses a high degree 
of sharpness and singular beauty and delicacy of tint. If at 
this instant it be thrown into water, it passes immediately to 
Prussian blue, losing at the same time, however, much of its 
sharpness, and sometimes indeed becoming quite blotty and 
confused. But if this be delayed, the picture, after attaining 
a certain maximum of distinctness, grows rapidly confused, 
especially if the quantity of liquid applied be more than the 
paper can easily and completely absorb, or if the brush in 
applying it be allowed to rest on, or be passed twice over 
any part. ‘The effect then becomes that of a coarse and ill- 
printed wood-cut, all the strong shades being run together, 
and a total absence prevailing of half lights. 
220. 'To prevent this confusion, gum-arabic may be added 
* Vulgarly, and in my opinion very conveniently and correctly so called, 
according to the true intent and meaning of Scheele. Trivial names for 
common objects are to be maintained and defended on principles far more 
general than systematic nomenclature. For this reason I trust never to 
see the name muriatic give way to hydrochloric, or nitric thrust aside for 
azotic acid. The prussic acid is that acid, whatever it be, which, united 
with oxide of iron as a base, forms Prussian blue, from which remarkable 
compound the whole history of cyanogen originated. The now ascertained 
existence of another ferrocyanate makes this recurrence to a trivial name 
for the yulgar one more necessary, 
