the Luminiferous ^Ether. 405 



compressibility at the surface of the sun, due to the weight of 

 an infinite column, and found it to be exceedingly small ; now 

 it may be possible that the expansion due to the excess of tem- 

 perature of a small fraction of one degree at the surface of the 

 sun over that at remote distances will diminish the density 

 as much, or about as much, as pressure increased it, thereby 

 making the density even more exactly uniform than it other- 

 wise would be. According to what we know of refraction, it 

 is impossible for a ray of light to be refracted in passing 

 through the aether only, — at least, not by a measurable 

 amount ; for not only are the density and elasticity practically 

 uniform, but their ratio is, if possible, even more constant as 

 shown by equations (16) and (160 • But the freedom of the 

 aether-molecules may be constrained, or their velocity impeded, 

 by their entanglement with gross matter, such as the gases 

 and transparent solids ; in which case refraction may be pro- 

 duced in a ray passing obliquely through strata of varying 

 densities. Neither is it believed that the aether does or can 

 reflect light ; for if it did, the entire sky would be more nearly 

 luminous. The rays in free space move in right lines. 



The masses of the molecules in different gases being inversely 

 as their specific heats, and as the specific heat of hydrogen is 

 3*4, and the computed mass of one of its molecules ~ 8 x 10~~ 29 * 

 of a pound, we have for the computed mass of a molecule of 

 the luminiferous aether, 



m = 18 x 10 29 X 46 x 10 u= 22 x 10 40 lb ' ' ' ^ 



* Stoney concludes that a it is therefore probable that there are not 

 fewer than something like a unit eighteen (10 18 ) of molecules in a cubic 

 millimetre of a gas at ordinary temperature and pressure" (Phil. Mag. 1868 

 [4] xxxvi. p. 141). According to the Kinetic theory the number of mole- 

 cules in a given volume under the same pressure and temperature is the 

 same for all gases. The weight of a cubic foot of hydrogen at the tempe- 

 rature of melting ice and under constant pressure being 0*005592 of a 

 pound, and as a cubic foot equals 28,315,000 cubic millimetres, the probable 

 mass of a molecule of hydrogen will be 



0-005592 _ 11 „ 



32 \ x 28,315,000 x 10 25 18 X 10 29 ' 



Maxwell gives — ^ of a gramme ==-- — — — s lb., which is about | the value 



given above (Phil. Mag. 1873 [4] xlvi.p.468). 



The ditference in these results arises chiefly from the calculated number 

 of molecules in a cubic foot of gas under ordinary conditions. Thomson gives 

 as the approximate probable number 17 X 10 25 , which is about J- the value 

 given by Stoney. Thomson's value would make the mass of a molecule 

 of aether about T V X 10 ~ 40 of a pound, which is not much different from 

 that found above. 



Phil. Mag. S. 5. Vol. 20. No. 126. Nov. 1885. 2 G 



