on Photographic Processes. 509 
deed, is there any feature in photography more general or 
more remarkable than the influence exercised by the presence 
of a certain degree of moisture in favouring the action of light, 
whether direct or indirect. 
238. There is this difference, however, in the excitement 
produced by simple water and by the mercurial solution, viz. 
that the latter is permanent, the former liable to fade; at least 
I have found this to be the case with the brown tinge produced 
by it in shade, though when blackened by a second exposure 
to sun no difference is perceived. On the other hand, when 
the nitrate is used, the brown hue frequently passes to abso- 
lute blackness without any subsequent exposure to sunshine ; 
and in that case the photographs produced have an intensity 
and opacity scarcely, if at all, inferior to that of printing 
ink. 
239. This high degree of opacity and depth, together with 
the comparative insensibility of the ground, is evidently ca- 
pable of being most usefully applied to the production of re- 
transfers. In fact, the photographs so produced being nega- 
tive are so far fitted for the purpose, and if used as models 
while in this, their transition state, and as it were self-fixed, 
so far from being injured by the transmission of light, they 
are actually acquiring additional sharpness and depth by every 
beam which passes. By seizing therefore the right point of 
dryness, and by using a very sensitive paper to receive the 
impression, there is no reason to doubt of success in procuring 
very perfect positive transfers. Some trials I have made have 
satisfied me as to the practicability of this, however contrary 
it may at first sight appear to the usual conditions of photo- 
graphy. 
240. In the positive cyanotype process, as improved by the 
addition of corrosive sublimate above recommended, we are 
furnished with another instance of a transformation effected 
by heat, analogous to those described in Art. 223. A picture 
prepared by this process, if heated, is transformed from posi- 
tive to negative and from blue to brown. On keeping the 
blue colour is restored, as well as the positive character. In 
Art. 224 I have referred this curious action to certain rays, 
a perfect silvering of all the sunned portions. Very singular and beautiful 
photographs, having much resemblance to Daguerreotype pictures, are thus 
produced ; the negative character changing by keeping, and by quite in- 
sensible gradations, to positive; and the shades exhibiting a most singular 
chatoyant change of colour from ruddy-brown to black when held more or 
less obliquely. No doubt also gold pictures with the metallic lustre might 
be obtained by the same process, though I have not tried the experi- 
ment.—J. I’. W. H. 
