RADIANT HEAT. 363 



report was not required immediately. But at the present date, being- 

 unwilling longer to delay, and rinding that, from the pressure of other 

 avocations, there is little probability of my being able to complete it, 

 rather than withdraw altogether, I am induced to ask the indulgence 

 of the Association for submitting only a portion of such a report. 



Prelimina ry remarks. 



Before commencing any analysis of recent investigations, it will be 

 necessary (for reasons which will become apparent) to take, in the 

 first instance, a very brief retrospective glance at certain fundamental 

 distinctions affecting the whole subject, long ago pointed out, but too 

 much overlooked by some writers since. It results from researches 

 pointed out in detail in the two former reports, that, under the some- 

 what wide term " radiant heat," several totally distinct species of 

 effect have been included. 



(I.) The simplest form of radiant heat, and most properly so called, 

 is that which arises simply from the cooling of a hot body, and which 

 emanates from terrestrial hot bodies of all temperatures, from those 

 which are the least elevated above that of the surrounding medium 

 up to the highest incandescence or combustion, and is distinguished 

 by two properties: 



(a.) A tendency to be absorbed by bodies in proportion to a certain 

 peculiarity of texture in their surface, but wholly independent of their 

 color. 



(&.) A total incapacity to pass by direct radiation through many 

 media, such as glass, &c, though transmissible freely through rock- 

 salt, and partially through certain others, called diathermanous media, 

 as found by the experiments of Melloni. 



(II.) At a certain stage of incandescence, other rays, also capable of 

 exciting heat, begin to be given off along with the former; these are 

 distinguished by the properties different from the former: 



(a.) A tendency to be absorbed by bodies in proportion to the dark- 

 ness of their color, or their absorption of light. 



(/?.) A power of transmissibility through all transparent substances, 

 without diminution through colorless media, and in various proportions 

 through colored media, according to their action on light. 



(y.) The power of exciting also the sense of vision, or being lumi- 

 niferous. 



This second species coexists in various proportions with the first, 

 and is most copious from the most intensely ignited bodies. 



(III.) Closely analogous to this last species, or identical with it, are 

 the heat ing rays of the sun, characterized by the same properties, 

 (a.,) (/?.,) and (y.,) and distinguished by being totally free from all 

 admixture of the first kind, (or Species I.)* 



* This result was formerly obtained by exposing together a blackened and a white- 

 washed thermometer to the solar rays, first with, and then without, a thick glass screen; 

 in this instance there was only the smallest difference in the absolute values, and none 

 whatever in the ratio of the rise of the two thermometers. In the brightest terrestrial rays 

 there is a marked and often very great difference in ratio as well as amount This consti- 

 tuted the ground of my conclusions as to the heterogeneity of the species of rays in the 

 latter case. 



