132 Action of the Spectrum on the Daguerreotype Plate. 
the other a very peculiar class, which are not only analogous 
to, but identical with, some at least of the rays conducted by 
and darted forth from obscurely hot metallic bodies,—which 
reside (at least in their greatest prevalence) in the less refran- 
gible portion of the spectrum (from the yellow éo, and perhaps 
somewhat beyond, the extreme red),—which possess chemical 
properties not participated in at all, or not in anything like so 
great a degree by the purely calorific rays which lie far be- 
yond the spectrum,—which as they exist where no light is 
manifested (as in hot mercury), are no way entitled to be 
called light (for against such a term as invisible light I must 
enter my protest),—and which as they lie in a region of 
the spectrum, where heat is known to exist abundantly, and 
produce there the chemical effects above spoken of, appa- 
rently by means of a peculiar kind of heat developed by 
them, are entitled to .be regarded as heat. To such rays 
(the existence of which I think I have demonstrated by expe- 
riments admitting of no other interpretation), the name para- 
thermic rays may, I conceive, be very properly assigned. 
And I mention the subject here in order pointedly to mark 
the difference between these rays, in my mode of conceiving 
them, and the Zithonic rays of Dr. Draper, which seem to be 
the photographic rays generally, with certain alleged additional 
properties which may very properly form the subject of further 
experimental inquiry. 
I have the honour to be, Sir, 
Your most obedient, 
Collingwood, Jan. 11, 1843. J. F. W. Herscue.. 
P.S. Added Jan. 20, 1843.—If we generalize our notion of 
radiating or Actinic influence so as to include the several 
phenomena of light, heat and chemical power operating 
molecular transformations, and conceive (as M. Becquerel 
seems disposed to do) that these several manifestations of such 
influence refer themselves rather to the powers, qualities and 
limits of the recipient than to original differences in the agent; 
(a view which may assuredly be taken, whether we regard that 
agent as an undulation mechanically propagated through a 
fluid, or in Sir William Hamilton’s more refined point of 
view, as an influence transmitted, wave-fashion, but yet without 
motion of material particles,) it will still be absolutely ne- 
cessary to have names distinctive of such very different forms 
of manifestation as heat, light, and chemical transformation, 
and rays will still continue to be spoken of as luminous, ther- 
mic, chemical, and perhaps by many more epithets, as science 
advances, without prejudice to any general views we may form 
as to causes, 
