RADIANT II 



But this last result he contends was simply clue to the greater prox- 

 imity of the source, and consequent greater differences of inclination 

 of the rays; and when equalized in this respect, the difference in 

 question disappeared, or might even be reversed. 



In the same notice Melloni refers briefly toother results which have 

 been obtained by means of rock-salt. 



(1.) That with a prism of rock-salt the maximum of caloric effect in 

 the solar spectrum is thrown further from the limit than with other 

 " prisms thermochroiques." 



(2.) That the radiation from the sun diminishes from the centre to 

 the circumference; that the radiation from the spots is less than from 

 the rest of the surface, and that of the equatorial region of the sun 

 greater; these results were obtained by M. Secehi. 



It may be right to add that I have been informed that in a work 

 entitled " Thermo -curose," not long since published at Naples, M. 

 Melloni has somewhat modified his former opinions, and seems disposed 

 to assent to the doctrine of the identity of the rays which produce 

 light and heat, or heat alone, according to their greater wave-length; 

 and has explained and reconciled some of his former difficulties. I 

 regret to be unable at present to give a more precise account. 



Mr. Draper also, I am informed, has been led to admit that the 

 chemical effects belong properly to the same set of rays, differing only 

 in the characteristic of peculiar wave-lengths. 



On the whole, the question of the evidence for and against this 

 theory is one eminently deserving of being fully discussed. I can only 

 pretend in this imperfect report to have suggested some of the ma- 

 terials which may assist in forming some judgment on this point, 



Analogies of Transmission of Light and Heat by Waves. 



The very important researches of Mr. Joule on heat and the dy- 

 namical theory to which they lead, though referring directly to heat 

 m its action on bodies as temperature or as latent heat, yet are not 

 without a bearing on the subject of radiant heat, as has been in some 

 degree pointed out in the excellent address of our President of last 

 year.— (Report, 1853, p. xlvii.) 



Mr. Joule's theory, though not as such dependent on the wave- 

 theory of heat, is yet eminently in accordance with it, and so far lends 

 it much support. If we suppose the temperature of a body to arise 

 from vibrations of its molecules, such vibrations may be excited in it 

 by the vibrations of an ethereal medium surrounding and penetrating 

 that mass of matter. In this respect the close analogy with sound is 

 preserved. These vibrations of heat, however, produce mechanical 

 changes m the constitution of the medium. They cause it to expand: 

 I e., they drive its molecules to greater distances apart; and when 

 carried to a certain extent, cause a fresh and sudden separation to a 

 tar greater extent, accompanied with a new arrangement of these 

 molecules, or a change of state in the body from solid to fluid or from 

 fluid to aeriform. Here the analogy with sound ceases to hold good, 

 except so far as that a temporary new arrangement of the molecules 



