210 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
weak solntion of hydriodate of potash. If there be any free chloride of gold present 
in the pores of the paper, it will be discoloured, the lights passing to a ruddy brown; 
but they speedily whiten again spontaneously, or at all events, on throwing it (after 
lying a minute or two) into fresh water, in which, being again rinsed and dried, it is 
now perfectly fixed. 
218 . If paper prepared as above recommended for the chrysotype, either with the 
ammonio-citrate or ammonio-tartrate of iron, and impressed, as in that process, with 
a latent picture, be washed with nitrate of silver instead of a solution of gold, a very 
sharp and beautiful picture is developed, of great intensity. Its disclosure is not in- 
stantaneous ; a few moments elapse without apparent effect ; the dark shades are then 
first touched in, and by degrees the details appear, but much more slowly than in the 
case of gold. In two or three minutes, however, the maximum of distinctness will 
not fail to be attained. The picture may be fixed by the hyposulphite of soda, which 
alone, I believe, can be fully depended on for fixing argentine photographs. 
219 . Cyanotype . — If a nomenclature of this kind be admitted (and it has some re- 
commendations), the whole class of processes in which cyanogen in its combinations 
with iron performs a leading part, and in which the resulting pictures are blue, may 
be designated by this epithet. The varieties of cyanotype processes seem to be innu- 
merable, 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 picture, if not so faint as to 
be quite invisible (and for this purpose 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 sharp- 
ness 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 
* 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 cya- 
nogen originated. The now ascertained existence of another ferrocyanate makes this recurrence to a trivial 
name for the vulgar one more necessary. 
