380 RADIANT HEAT. 



is occasioned by the sonorous vibrations. The transmission of livmi- 

 niferous waves has a velocity which, though enormous, is capable of 

 measurement. Whether that of the longer non-luminiferous but 

 caloriferous waves is the same has not been, I believe, experimentally 

 verified, but must theoretically be supposed the same, unless, indeed, 

 it be only approximately the same for waves within the narrow limits 

 of the luminiferous scale, and diverge from that value beyond those, 

 limits. 



Again, the passage or process of the vibrations in a body receiving 

 heat is slow ; to this conduction of heat there does not seem to be any- 

 thing strictly analogous in sound. In general the passage of light 

 through transparent bodies excites in them no vibrations capable of 

 affecting our eyes with the sense of light; i. e., the medium does not 

 become luminous, unless we except the case of the phosphorescence 

 of fluor-spar and some other bodies after exposure to light. So far, 

 indeed, as the transparency is imperfect, and in all opaque bodies the 

 vibrations which constitute light are stopped, or changed in such a 

 manner that they give rise to vibrations in the body constituting heat, 

 just as those longer vibrations do which constitute that species of 

 radiation which is derived from the mere cooling of a hot body; but 

 this does not occur in transparent bodies. It would seem to be the 

 law that if a ray, or a series of waves of the proper length to be 

 luminiferous, impinge on an opaque body, they communicate vibra- 

 tions to its molecules, which again transmit to the surrounding ether 

 other waves of greater length, which in like manner traverse space and 

 can again excite vibrations in bodies on which they impinge; or if 

 from any source a body have internal vibrations of a certain intensity, 

 (whether forming leaves, or of what lengths, we have no means of 

 deciding.) it can transmit to the surrounding ether vibrations which 

 constitute waves of lengths greater than a certain given length, viz: 

 that which belongs to the deepest red luminiferous rays. If its 

 internal vibrations are increased in intensity beyond a certain point, 

 it then acquires the power of communicating (in addition to the last) 

 other vibrations to the ether forming waves of other and smaller 

 lengths, so as to give rise to light. 



Origin of the Solar Heed. Professor W. Thomson's Theory. 



Some very important speculations have been brought forward on 

 the source, and thus bear on the nature, of the solar heat, by Professor 

 Thomson"", in immediate connexion with the theory of Mr. Joule, and 

 on the principle that the energy of the heat thus emitted must be 

 accompanied by an equivalent expenditure of mechanical force. On 

 this principle he institutes numerical calculations, the main results of 

 winch, together with a brief exposition of the principles, may be 

 given as follows : 



The kind of force acting, or the source of solar heat, the author 

 conceives may be expressed by several hypotheses, each of which he 

 examines: 



^Transactions of the Royal Society of Edinburgh, vol. sxi, part 1. On the Mechanical 

 Energies of the Solar System. 



