364 RADIANT HEAT. 



When by artificial means the luminiferous rays are analysed, some 

 rays are found in which great heating- power coexists with feeble 

 illuminating power; but under the same conditions and for the same 

 ray the heating is probably directly proportional to the illuminating 

 intensity. 



In this way it was that from the distinctions obtained in my original 

 experiments, (1825,) I was led to describe generally the communica- 

 tion of heat as effected in three distinct ways: 



(1.) By conduction (including what some term convection.) 



(2.) By radiation (in the ordinary sense of the term) or mere- cool- 

 ing of a hot body, (Species I above.) 



(3.) By the agency of light, whether from the sun or flames, (Spe- 

 cies II and III.) 



In the first instance, it was natural to regard these as so many 

 distinct kinds of physical action producing heat, but more recent 

 researches, especially those of Professor Forbes, have enabled us to 

 connect them by the simple and uniform analogy supplied by the un- 

 dulatory hypothesis. If we adopt the hypothesis of undulations of 

 decreasing lengths, those of the greatest length correspond to the 

 Species I, these continue to be given out as the temperature is raised, 

 or as combustion proceeds more intensely, along with others of suc- 

 cessively less wave-length, until we arrive at Species II, or those 

 which are of the proper length to affect also the retina with the sense 

 of vision, and at last the wave-lengths are too small to produce either 

 luminiferous or calorific effects; but here they seem to obtain their 

 maximum of chemical action. But rays of all wave-lengths thus con- 

 tinue to be given out simultaneously. They all produce or excite heat, 

 more or less, when stopped or absorbed; and this probably dependent, 

 in some direct ratio, on the greater wave-length simply; while the illu- 

 minating effect depends on some peculiar relation to certain wave- 

 lengths only determined by some physiological conditions of the retina 

 at present unknown. Substances which are transparent transmit 

 freely rays of the visual wave-lengths, which of course carry with 

 them their heating powers. 



Opaque substances which are diathermanous transmit in the same 

 way rays of the longer wave-lengths, but not those of shorter. 



The action of the texture of surfaces seems purely mechanical, and 

 probably influences the absorption of all rays, but its full effect is 

 produced on the rays of longer wave-length; while that of color is 

 purely optical and applies to rays of luminiferous wave-lengths only. 



Refraction, polarization, interference, and the like properties of 

 light, it would be easily seen, must be accompanied with just such 

 indications of heating effect as might consist with the modification 

 Avhich the light in the respective cases might undergo. If the light 

 were extinguished in certain conditions of polarization, or of inter- 

 ference, the heat would of course disappear with it, and the changes 

 of intensity would be similar; that is, as regards either the solar rays 

 (III) or those of species (II); and consequently, in the case of lumi- 

 nous hot bodies, in which Species I and II coexist, the effects in ques- 



