86 MOLECULAR MOTION AND ITS ENERGY 37 



only seldom be overtaken by those that follow, and will stop 

 them but little. The stream of molecules therefore flows 

 into a less dense gas almost as it would into vacuum. 



But even if the pressure outside the containing vessel is 

 not much less than that within, Graham's law still holds 

 good. In this case the distribution of pressure in the 

 orifice and its immediate neighbourhood is certainly quite 

 different from that which accompanies efflux into vacuum. 

 But if experiments are made with two different gases with 

 the same two values of the pressures within and without, 

 the pressures in the orifice itself will also be the same in 

 both cases. The equation 



remains correct therefore in this case too. In like manner 

 also, for the same reasons as in the former case, the speed 

 of efflux is proportional to the molecular speed. Therefore 

 Graham's law, which was deduced before from these two 

 assumptions the law, namely, that the times of efflux of 

 two gases are as the square roots of the specific gravities 

 must hold good also for efflux into space already containing 

 the gas. And if the space contains a different gas the law 

 holds good in the same way. 



This law of effusion is to such a degree confirmed by 

 experiment that Leslie, 1 and later Bun sen, 2 have been 

 able to found on it a method of determining specific 

 gravities. Column I. of the subjoined table gives the values 

 of the specific gravities determined by Bun sen by this 

 method, and column II. those that have been calculated by 

 Gay-Lussac's law ( 30) from the atomic weights. 



1 Experimental Inquiry into the Nature and Propagation of Heat, 1804, 

 p. 534 ; Gilb. Ann. xxx. 1808, p. 260. 



2 Gasometrische Methoden, Braunschweig 1857, p. 127 ; 2nd ed. 1877, p. 184. 



3 [This is the mixture of hydrogen and oxygen produced by electrically 

 decomposing water. TB.] 



