the Luminiferous JEther. 391 



is inconsistent with the uniform elasticity and density of the 

 aether which it is believed to possess; and every consideration 

 would lead one to accept the lowest density consistent with 

 those qualities which would enable it to perform functions 

 producing known results. 



In a work on the i Physics of iEther/ by S. Tolver Preston, 

 it is estimated that the probable inferior limit of the tension 

 of the aether is 500 tons per square inch, a very small value 

 compared with that of HerschePs. But the hypothesis 

 upon which this author founded his analysis was — The tension 

 of the aether exceeds the force necessary to separate the atoms 

 of oxygen and hydrogen in a molecule of water ; as if the 

 atoms were forced together by the pressure of the aether, as 

 two Magdeburg hemispheres are forced together by the external 

 air when there is a vacuum between them. This assumption 

 is also gratuitous, and is rejected for want of a rational 

 foundation. 



Young remarks: — "The luminiferous aether pervading all 

 space is not only highly elastic, but absolutely solid."* We 

 are not certain in what sense this author considered it as solid ; 

 but if it be in the sense that the particles retain their relative 

 positions, and do not perform excursions as they do in liquids, 

 it is a mere hypothesis, which may or may not have a real 

 existence. If it be in the sense that the particles suffer less 

 resistance to a transverse than to a longitudinal movement, 

 there are some grounds for the statement, as shown in 

 circularly-polarized light. Bars of solids are more easily 

 twisted than elongated, and generally the shearing-resistance 

 is less than for a direct stress. It certainly cannot be claimed 

 that the compressibility of the aether (in case we could capture 

 a quantity of it) is less than that of solids. 



Sir William Thomson, in making a probable estimate of the 

 density of the aether, made a more plausible hypothesis, by 

 assuming that " the maximum displacement of the molecules 

 of the aether in the transmission of heat-energy was ^ of a 

 wave-length of light, the average of which may be taken as 

 goQQQ of an inch." Hence the displacement was assumed to 

 be 2 50 q 000 of an inch; by means of which he found the weight 

 of a cubic foot to be § x 10" 20 of a poundf. We also notice 

 that one Belli estimated the density of the aether to be \ X 10~ 13 

 of a pound J; but M. Herwitz, assuming this value to be too 

 small and Thomson's as too large, arbitrarily assumed it as 



* Young's Works, vol. i. p. 415. 

 t Phil. Mag. 1855 [4] ix. p. 39. 

 % Cf. Fortschritte der Physik, 1859. 

 2F2 



