Prof. Norton on Molecular Physics. 95 



take place; while the motions which constitute light are not 

 capable of producing such changes. If it does act upon the 

 material particles of bodies, its action is at any rate essentially 

 different from that of heat ; for light does not cause such a sepa- 

 ration of the material particles as to produce expansion. 



As the temperature of a body rises, the number of heat- colours 

 which it gives out increases, and at a certain temperature it 

 begins to radiate light, at first red light, which has the greatest 

 wave-length \ and as the temperature becomes still higher, new 

 waves are added until the body appears white-hot. The appear- 

 ance of new wave-lengths appears to depend upon the fact that, 

 with the increased expansion of the body, or, what is the same 

 thing, with the increased distances between its material particles, 

 the particles of aether which intervene between these, and per- 

 haps also the material particles themselves, are able to make 

 continually new and more rapid movements. 



Notwithstanding the difference between heat and light, the 

 question may be raised whether both effects are not produced by 

 one and the same source, something in the same way as electri- 

 city and magnetism result from a common force — or, to express 

 it in another way, whether the same movements are capable of 

 rendering themselves perceptible both as light and as heat. 



In the case of light, we know that it consists of transverse vi- 

 brations ; and the polarization of heat necessitates the conclusion 

 that in it also transverse oscillations are present. It is likewise 

 quite possible that our eye, in consequence of its peculiar nature, 

 may perceive, as extremely great, alterations of these transverse 

 oscillations which are not capable of producing perceptible ther- 

 mal changes, so that the intensity of light of the same wave- 

 length may become very great, without our being able to detect 

 a change of heating-effect. But whether the same movement is 

 in reality perceived by us both as light and as heat, is a point 

 which requires further investigation. 



XIII. On Molecular Physics. By Prof. W. A. Norton*. 

 [Continued from vol. xxviii. p. 433.] 



IN a former part of this paper a succinct exposition has been 

 given of a consistent general theory of Molecular Forces, 

 and the Molecular Constitution of bodies, and special theories of 

 the different states of aggregation of matter, and the processes of 

 transformation from one state to another, as well as of the essen- 

 tial nature and modes of excitation and propagation of the two 

 agencies of light and heatf. 



* From Silliman's Journal for May, 1865. 



t In a paper " On Heat-vibrations/' by Mr. James Croll, published in 



