1920] The Theory of Relativity 27 



nomena of optics, electrostatics, etc., attempts were made to measure 

 its properties. Newton thought its density was something like 700,000 

 times less than that of water ; Lord Kelvin thought the ether so attenu- 

 ated that it had a density of only one-quintillionth that of water, and 

 many other scientists agreed with them as to the extreme tenuity of 

 the ether. On the other hand, Sir J. J. Thomson says that "all mass 

 is mass of the ether ; all momentum, momentum of the ether ; and all 

 Kinetic energy, kinetic energy of the ether," which shows the confi- 

 dence of the foremost English physicist in the reality of the ether, 

 but he goes on to say that this view "requires the density of the ether 

 to be immensely greater than that of any known substance." Other 

 leading physicists take this view also, and Sir Oliver Lodge insists 

 that the ether has a mass of one quadrillion grams per cubic centi- 

 meter, and that "compared to ether the densest matter, such as lead 

 or gold, is a filmy gossamer structure like a comet 's tail ' ' ! 



Now we all agree, I am sure, that when Kelvin claimed that one 

 cubic centimeter of the ether weighs one-quintillionth of a gram, and 

 Lodge says it weighs one quadrillion grams, (which is one million 

 tons), there is a serious discrepancy, to say the least, in the estimates 

 of physicists as to the properties of the ether, and we are not sur- 

 prised to learn that there are a multitude of ether theories, — solid, 

 liquid, elastic, labile, irrotational, gyrostatic, adynamic, etc., so no 

 wonder Relativity wants to get rid of the whole thing by turning the 

 ether out of doors. 



And yet there comes, in protest, the cool reasoning of one of our 

 very foremost American physicists, Millikan, who says that the ether 

 "was called into being solely for the sake of furnishing a carrier for 

 electromagnetic waves, and it obviously stands or falls with the exist- 

 ence of such waves in vacuo, and this has never been questioned by 

 anyone, so far as I am aware." Other leading physicists protest also 

 against giving up the notion of the ether for what Newton called "the 

 forlorn idea" of empty space, among them being Lorentz, the great 

 Dutch mathematician and physicist. 



But Planck, the originator of the Quantum Theory, gives up the 

 ether, and Einstein of course does the same, for his first postulate 

 implies this very thing. But his second postulate states that light in 

 a vacuum is propagated with a constant velocity quite independent 

 of the velocity of its source, and queerly enough, this postulate seems 

 to assume the existence of the ether, which the first postulate denies. 



