74 ABSORPTION OF CALORIC BY THE AIR. 



Experienee has proved that the conditions of the 

 sun's rays are not always the same ; and there are few 

 persons who have not observed that a more than usual 

 scorching influence prevails under some atmospheric 

 circumstances. This is also evidenced in the effects 

 produced on the foliage of trees, which, though often 

 attributed to electricity, is evidently due to heat. An 

 examination of the solar radiations, as exhibited in the 

 prismatic spectrum, has proved the existence of a class 

 of heat rays, which manifest themselves by a very pecu- 

 liar deoxidizing power quite independent of their caloric 

 properties, to which the name of parathermic rays 

 has been given.* We are protected from the severe 



Atmosphere, and the law of extinction of the solar rays in passing 

 through it, by James D. Forbes, Esq., F.R.S., &c., will be found a 

 most complete investigation of this subject 



The experiments were, for the most part, made in Switzerland 

 with Sir John Herschel's actinometer, and they prove satis- 

 factorily, "That the absorption of the solar rays by the- strata 

 of air to which we have immediate access, is considerable in 

 amount for even moderate thicknesses.'' 



* After referring to several curious and instructive experiments, 

 in which peculiar chemical changes are produced under the influ- 

 ence of the solar rays by their HEAT, Sir John Herschel says : 



" These rays are distinguished from those of Light by being 

 invisible ; they are also distinguished from the pure calorific rays 

 beyond the spectrum, by their possessing properties (of a peculiar 

 character, referred to in former papers} either exclusively of the 

 calorific rays, or in a 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 ones. 

 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 

 distinction, I would propose the term parathermic rays to designate 

 them. These are the rays which I conceive to be active in pro- 

 ducing those singular molecular affections which determine the 

 precipitation of vapours in the experiments of Messrs. Draper. 

 Moser, and Hunt, 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 epi- 

 polic forces." On certain improvements in Photographic Processes, de- 



