ETHER. 773 



a body having three unequal axes. The specific dielectric capacity for these 

 axes are respectively 



4773 3-970 3-811 



and the squares of the indices of refraction 



4-576 3-886 3'591 



Physical constitution of the (ether. What is the ultimate constitution of the 

 aether ? is it molecular or continuous ? 



We know that the aether transmits transverse vibrations to very great 

 distances without sensible loss of energy by dissipation. A molecular medium, 

 moving under such conditions that a group of molecules once near together 

 remain near each other during the whole motion, may be capable of trans- 

 mitting vibrations without much dissipation of energy, but if the motion is 

 such that the groups of molecules are not merely slightly altered in configura- 

 tion but entirely broken up, so that their component molecules pass into new 

 types of grouping, then in the passage from one type of grouping to another 

 the energy of regular vibrations will be frittered away into that of the irregular 

 agitation which we call heat. 



We cannot therefore suppose the constitution of the aether to be like that 

 of a gas, in which the molecules are always in a state of irregular agitation, 

 for in such a medium a transverse undulation is reduced to less than one five- 

 hundredth of its amplitude in a single wave-length. If the aether is molecular, 

 the grouping of the molecules must remain of the same type, the configuration 

 of the groups being only slightly altered during the motion. 



Mr S. Tolver Preston* has supposed that the aether is like a gas whose 

 molecules very rarely interfere with each other, so that their mean path is far 

 greater than any planetary distances. He has not investigated the properties 

 of such a medium with any degree of completeness, but it is easy to see that 

 we might form a theory in which the molecules never interfere with each 

 other's motion of translation, but travel in all directions with the velocity of 

 lio-ht ; and if we further suppose that vibrating bodies have the power of im- 

 pressing on these molecules some vector property (such as rotation about an 

 axis) which does not interfere with their motion of translation, and which is 

 then carried along by the molecules, and if the alternation of the average 



* Phil. Mag., Sept. aud Nov. 1877. 



