ON PHOTOGRAPHIC PROCESSES. 
O 
cability of this, however contrary it may at first sight appear to the usual conditions 
of photography. 
240. In the positive cyanotype process, as improved by the addition of corrosive 
sublimate above recommended, we are furnished with another instance of a trans- 
formation effected by heat, analogous to those described in Art. 223. A picture pre- 
pared by this process, if heated, is transformed from positive 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, which, whether they 
be regarded as rays of heat, or light, or of some influence, sui generis, accompany in 
the spectrum the red and orange rays, and are also copiously emitted by heated 
bodies short of redness. These rays are distinguished from those of light by being 
invisible ; they are also distinguished from the purely calorific rays beyond the spec- 
trum by their possessing the properties recorded in Arts. 160, 223, either exclusively of 
the calorific rays, or in a very much higher degree. They may perhaps not improperly 
be regarded as bearing the same relation to the calorific spectrum which the photogra- 
phic rays do to the luminous one, and if the restriction to these rays of the term 
thermic as distinct from calorific be not (as I think in fact it is not) a sufficient di- 
stinction, I would propose the term parathermic rays to designate them. These are 
the rays (if I may indulge in speculation which I propose to bring to the test of ex- 
periment hereafter) which I conceive to be active in producing those singular mole- 
cular affections which determine the precipitation of vapours in the experiments of 
Messrs. Draper, Moser, and Httnt, and which will probably lead to important dis- 
coveries as to the intimate nature of those forces resident on the surfaces of bodies to 
which M. Dutrochet has given the name of epipolic forces. These also, I cannot 
help considering it as highly probable, are the rays which radiated from molecule to 
molecule in the interior of bodies, determine the discharge of vegetable colours at the 
boiling temperature (see Art. 162), and the innumerable isomeric and other atomic 
transformations of organic bodies which take place at temperatures below redness. 
The term latent light, I confess, carries with it to my mind no distinct conception ; 
still less capable of being introduced into scientific language appears such a term as 
invisible light. Whether the rays to which such terms have been applied shall or 
shall not turn out, on inquiry, to be identical with my “parathermic” rays, can only 
be decided by experiments to be instituted for that purpose; but at all events I feel 
strongly disposed to insist on the distinction between these rays and those of pure 
heat, and in referring them to a peculiar region of the spectrum (though without 
denying their more sparing distribution over every other part of it), I consider them 
at all events as sufficiently identified by their characters, there eminently developed, 
to become legitimate objects of scientific discussion. 
241. The action of the calorific rays, as such, demonstrated by the rapidity of eva- 
poration of water or alcohol which takes place under their influence, is traced (in 
addition to the facts brought forward in the notes on my first paper on this subject) 
