Prof. Norton on Molecular Physics. 429 



in the spheroidal state of liquids evidence of such action exerted 

 by the radiant heat from the hot vessel upon the liquid resting 

 upon its interior surface ? 



It is only when the heat-waves impinge upon isolated parti- 

 cles, or a small group of particles, that a progressive motion 

 should be imparted. This supposition is apparently realized in 

 the case of cometary matter repelled by the sun*. 



Light. 



The question of the precise relation which the two physical 

 agents, light and heat, bear to each other, has not been defini- 

 tively settled; but the weight of evidence preponderates very 

 decidedly in favour of the doctrine of their essential identity. 

 The only "formidable outstanding objection" to this view con- 

 sists in the fact that a strong light may be obtained which has 

 little, if any, heating power f. According to Melloni, the green- 

 ish light obtained by transmission of white light through a 

 peculiar species of green glass coloured by oxide of copper, 

 " exhibits no calorific action capable of being rendered perceptible 

 by the most delicate thermoscopes, even when it is so concen- 

 trated by lenses as to rival the direct raysof the sun in brilliancy." 

 May it not be that the explanation of the possible existence of 

 light without heat, thus made out, is to be found in the presence, 

 in the luminous beam, of a certain number of radiations which 

 have individually too feeble an intensity to exercise any calorific 

 action upon bodies, but are still capable of producing a sensible 

 impression upon the organ of vision ? Upon the theory of the 

 constitution of molecules adopted in the present paper, we may 

 suppose that vision results from some action of the luminous 

 pulses upon the molecular atmospheres merely, while heat-expan- 

 sion is not produced unless the individual pulses have sufficient 

 intensity to penetrate to the surface of the central atoms of the 

 molecules. According to this idea, as rays that penetrate to 

 different depths in the molecular atmospheres of a medium 

 should be unequally absorbed, the rays of feeble intensity or of 

 pure light may be separated from those which, by reason of their 



* In the article by the author, "On the Theoretical Determination of 

 the Dimensions of Donati's Comet" (see Silliman's Journal, vol. xxxii. 

 No. 94. p. 54, &c), it is established by rigorous calculation that the par- 

 ticles of matter disseminated over the breadth of the tail of the comet were 

 exposed to a force of repulsion from the sun, of various degrees of inten- 

 sity, between two ascertained limits. In the light of the theoretical views 

 now offered, we must conclude that the matter thus unequally repelled 

 consisted of particles of different sizes, or different absorptive powers for 

 heat, or of different-sized groups of particles. 



f See Report of Rev. Baden Powell, M.A., F.R.S., for 1854, on Radiant 

 Heat, published in the Smithsonian Report for 1859, p. 368. 



