ETHER. 767 



have been observed, and the wave-lengths measured, in the case of radiations, 

 which can be detected only by their heating or by their chemical effects. 



Elasticity, tenacity, and density of the ceiher. Having so far determined the 

 geometrical character of the process, we must now turn our attention to the 

 medium in which it takes place. We may use the term sether to denote this 

 medium, whatever it may be. 



In the first place, it is capable of transmitting energy. The radiations which 

 it transmits are able not only to act on our senses, which of itself is evidence 

 of work done, but to heat bodies which absorb them ; and by measuring the 

 heat communicated to such bodies, the energy of the radiation may be calculated. 



In the next place this energy is not transmitted instantaneously from the 

 radiating body to the absorbing body, but exists for a certain time in the 

 medium. 



If we adopt either Fresnel's or Maccullagh's form of the undulatory theory, 

 half of this energy is in the form of potential energy, due to the distortion of 

 elementary portions of the medium, and half in the form of kinetic energy, 

 due to the motion of the medium. We must therefore regard the aether as 

 possessing elasticity similar to that of a solid body, and also as having a finite 

 density. If we take Pouillet's estimate of 17633 as the number of gramme- 

 centigrade units of heat produced by direct sunlight falling on a square centi- 

 metre in a minute, this is equivalent to 1'234 x 10" ergs per second. Dividing 

 this by 3'004 x 10 10 , the velocity of light in centimetres per second, we get for 

 the energy in a cubic centimetre 4'1 x 10~" ergs. Near the sun the energy in 

 a cubic centimetre would be about 46,000 times this, or T886 ergs. If we 



further assume, with Sir W. Thomson, that the amplitude is not more than one 



n i 



hundredth of the wave-length, we have Ap= . Q -, or about ; so that we have- 

 Energy per cubic centimetre =|/>F 2 ^y = 1'886 ergs.* 



Greatest tangential stress per square centimetre = pVAp =30'176 dynes. 



Coefficient of rigidity of ether = />F 2 = 842'8. 



Density of sether = p =9'36xlO- I . 



The coefficient of rigidity of steel is about 8 x 10", and that of glass 

 2-4 x 10". 



* [The numbers in this column are incorrectly deduced from the data. They should be 1-886, 

 60-352, 965-632 and 1-07 x HT 18 .] 



