510 On certain Improvements on Photographic Processes. 
which, whether they be regarded as rays of heat, or light, or 
of some influence, swz 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 spectrum 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 
photographic rays do to the luminous one, and if the restriction 
to these rays of the term thermic as distinct from calorific be 
not (as [ think in fact it is not) a sufficient distinction, 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 experiment hereafter) which I conceive 
to be active in producing those singular molecular affections 
which determine the precipitation of vapours in the experi- 
ments of Messrs. Draper, Moser, and Hunt, and which will 
probably lead to important discoveries as to the intimate na- 
ture 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 scien- 
tific 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 ¢hese rays and those of pure 
heat, and in referring them to a peculiar region of the spec- 
trum (though without denying their more sparing distribu- 
tion 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 dis- 
cussion. 
241. The action of the calorific rays, as such, demonstrated 
by the rapidity of evaporation 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 sub- 
