' RADIANT HEAT. 361 



all these important discoveries (besides others of minor value) are 

 imperishably associated with the name of Melloni. Our own country 

 as fairly and incontestably boasts, besides improvements in the appa- 

 ratus and methods, many important results connected with the trans- 

 mission of heat, accurate measures of its refraction, together with 

 some indication of phenomena analogous to those of diffraction. In 

 addition to these, the sole and undisputed credit of first unequivocally 

 establishing ihe grand facts of the polarization of heat, even from non- 

 luminous sources, by transmission through mica, through tourmaline, 

 and by reflection; together with the peculiar and invaluable property 

 of mica split by sudden heating, (a fact holding a parallel rank with 

 that of the diathermancy of rock-salt;) the dipolarization of heat; its 

 consequent double refraction and interference; its circular and elliptic 

 polarization; its length of .wave, and the production of that wave by 

 transverse vibrations; the confirmation of the circular polarization by 

 the rock-salt rhomb, and the peculiar effects of metallic reflection; 

 these constitute the unquestionable claims of Professor Forbes. 



On the main point in controversy between these two philosophers, 

 the equal or unequal polarizability of heat from different sources, I 

 have endeavored to place the facts and arguments clearly before the 

 reader; but must confess my own conviction to be in favor of the 

 unequal ratio of polarizability in the radiations from luminous and from 

 obscure sources, while in some instances the apparently opposite 

 results seem distinctly traced to known causes, and in others the 

 equalization of the effects appears to depend on some of those modifi- 

 cations which the intervention of screens produces in the nature of 

 the rays of heat. 



The very remarkable class of phenomena just referred to is, per- 

 haps, of all the recent discoveries, that which seems most singular 

 and anomalous. That the same ray should acquire an entire change 

 of property and nature by and in the act of simply passing through 

 certain media seems little in accordance with any conception we can 

 form of such radiation. Is this, we may ask, a real change of consti- 

 tution, or is it a separation or analysis of the ray into its components ? 



I have elsewhere remarked that the terms "luminous' 7 and "dark" 

 heat are of somewhat barbarous appearance; and the objection is more 

 than etymological, especially as we now find the luminosity of the 

 source is not the essential characteristic of the qualities of the rays. 

 And again, in the compound radiation from luminous sources there is 

 included a considerable portion of "dark" heat as disclosed by its 

 relation to surfaces in absorption. 



The relations of heat to surfaces in absorption, and in the corres- 

 ponding inverse effects of radiation, are among the most important 

 portions of the subject, and I have in consequence been desirous to 

 draw particular attention to the very valuable investigations of Presi- 

 dent Bache. 



The properties which characterize the different species of heat (as 

 we have seen) have been most remarkably developed and principally 

 studied in the phenomena of transmission. A wide field is open to 

 the experimenter in connecting these properties with those belonging 



