84 



MOLECULAR MOTION AND ITS ENERGY 



37 



Maxwell's and Avogadro's laws hold good for both. If 



both gases are under the same pressure p, the two equations 



p = iw^lV, p = i7rp 2 O 2 2 



hold good, pu p 2 being the densities of the two gases, and 

 ftp I"1 2 the mean speeds of their molecules. If, as in 28, 

 we refer the density not to that of water, but to that' of air 

 D, the formulae take the form 



P = jwDsA 2 , p = jTritejiy, 



and contain only the magnitude D which varies with p, 

 since the specific gravities s l} s 2 are independent of pressure 

 and temperature. These formulas therefore give 



2 _ 



and consequently 



or the molecular speeds are inversely as the square roots 

 of the specific gravities. In like manner the speeds with 

 which different gases stream out into vacuous space are 

 inversely as the square roots of the specific gravities of the 

 gases. The times required for efflux of equal volumes of 

 different gases are therefore directly proportional to the 

 square roots of the specific gravities of the gases, these 

 volumes being measured under the same pressure. 



This law is confirmed by observation, in proof of which 

 I subjoin some of Graham's 1 observed times of efflux 

 of different gases, together with Begnault's and Henry's 

 determinations of the specific gravities which he used. 



Phil. Trans. 1846, p. 573. 



