390 Royal Society : — Mr. J. C. Maxwell on Double 



tetrad or hexad metals. May not this be explained by the plas- 

 ticity of the metalloids in question ? 



"May we not from these ideas be justified in denning a metal, 

 provisionally, as a substance the absorption-spectrum of which is 

 generally the same as the radiation spectrum, while the metalloids 

 are substances the absorption-spectrum of which, generally, is not 

 the same ? 



" In other words, in passing from a hot to a comparatively cold 

 state, the plasticity of these latter comes into play, and we get 

 a new molecular arrangement. Hence are we not justified in 

 asking whether the change from oxygen to ozone is but a type of 

 what takes place in all metalloids ?" 



Dec. 18. — Joseph Dalton Hooker, C.B., President, in the Chair. 



The following communication was read : — 



" On Double Refraction in a Viscous Pluid in motion. By J. 

 Clerk Maxwell, M.A., F.R.S. 



According to Poissous * theory of the internal friction of fluids, 

 a viscous fluid behaves as an elastic solid would do if it were pe- 

 riodically liquefied for an instant and solidified again, so that at 

 each fresh start it becomes for the moment like an elastic solid free 

 from strain. The state of strain of certain transparent bodies may 

 be investigated by means of their action on polarized light. This 

 action was observed by Brewster, and was shown by Fresnel to be 

 an instance of double refraction. 



In 1866 I made some attempts to ascertain whether the state of 

 strain in a viscous fluid in motion could be detected by its action 

 on polarized light. I had a cylindrical box with a glass bottom. 

 Within this box a solid cylinder could be made to rotate. The 

 fluid to be examined was placed in the annular space between 

 this cylinder and the sides of the box. Polarized light was thrown 

 up through the fluid parallel to the axis, and the inner cylinder 

 was then made to rotate. I was unable to obtain any result with 

 solution of gum or sirup of sugar, though I observed an effect 

 on polarized light when I compressed some Canada balsam which 

 had become very thick and almost solid in a bottle. 



It is easy, however, to observe the effect in Canada balsam, which 

 is so fluid that it very rapidly assumes a level surface after being 

 disturbed. Put some Canada balsam in a wide-mouthed square 

 bottle ; let fight, polarized in a vertical plane, be transmitted 

 through the fluid ; observe the light through a Nicol's prism, and 

 turn the prism so as to cut off the light ; insert a spatula in the 

 Canada balsam, in a vertical plane passing through the eye. When- 

 ever the spatula is moved up or down in the fluid, the light re- 

 appears on both sides of the spatula ; this continues only so loug as 

 the spatula is in motion. As soon as the motion stops, the light 

 disappears, and that so quickly that I have hitherto been unable to 

 * Journal de 1'fCcole Polytechnique, tomexiii. cah. xx. (1829). 



