Prof. Norton on Molecular Physics. 431 



In order that a ray may be completely absorbed, it must en- 

 counter a layer of the electric atmosphere of a molecule which 

 pulsates naturally in unison with the wave-pulsation of the ray. 

 The absorbing action of a molecule should therefore depend upon 

 the physical condition of its atmosphere as to rates of pulsation, 

 density, &c, and also upon the comparative extent of its electric 

 and sethereal atmospheres. For example, a medium would per- 

 mit the free passage of any ray that did not penetrate to the 

 surface of the electric atmospheres of its molecules. On the 

 other hand, a medium would intercept rays that should pene- 

 trate to atmospheric layers that afe in unison with the rays. 

 Accordingly, if an incandescent vapour should emit rays of cer- 

 tain colours, as shown by bright bands in its spectrum, those 

 colours, if transmitted through the vapours, should be absorbed, 

 and the spectrum given by transmitted light should show dark 

 lines answering to the bright lines of the other spectrum — which 

 is the well-known discovery of Kirchhoff and Bunsen. 



According to the theory of crystallization presented on p. 384 

 &c, in all the systems of crystallization in which the axes of 

 molecular attraction are unequal, the electric atmospheres of the 

 molecules are compressed unequally in the lines of direction of 

 these axes. Now, if these atmospheres are compressed unequally, 

 the same will be true of the sethereal atmospheres which pervade 

 them. Thus in all forms of crystals which have unequal axes, 

 the sethereal atmospheres of its separate particles will have un- 

 equal densities in the directions of the molecular axes. It is 

 well known that all such crystals have the property of double 

 refraction, and that this property is attributed to a supposed in- 

 equality of density or of elasticity of the aether in the direction 

 of certain molecular axes. A mechanical pressure exerted along 

 a certain line or plane also developes the property of double 

 refraction in ordinary refracting media; and such compression 

 should give rise to an increased density of the sethereal atmo- 

 spheres along the line of pressure. Accordingly our general 

 theory of the constitution of molecules and of molecular forces 

 conducts to the physical basis assumed, in the undulatory theory 

 of light, in explanation of double refraction. 



The phenomenon of "atomic circular polarization " by liquids, 

 discovered by Biot, who established that the effect depended 

 solely upon the number of atoms encountered by the light, what- 

 ever may be the density of the medium — and the phenomenon 

 of " magnetic circular polarization," in which the direction of 

 rotation of the plane of polarization corresponds with that of 

 the revolution of the circular magnetic currents — are decided in- 

 dications that optical phenomena result mainly from the action of 

 the atmospheres of molecules upon the rays of light. Nume- 



