Effusion of Argon, Helium, and some other Gases. 425 



observed by Saint- Venant and Wantzel, and came to the 

 conclusion that the theory was incorrect. In the same year, 

 however, Hirn's experimental results were very fully re- 

 calculated by Hugoniot*, who showed that they were not in 

 discord with the theory when the latter was extended in some 

 important respects. The experiments of Wilde led Osborne 

 Reynolds f to reinvestigate the theory, and he arrived, prac- 

 tically simultaneously with Hugoniot, at the same important 

 extension of the theory. Quite recently the efflux of gases 

 has been the subject of an elaborate memoir by Parenty J, who 

 rejects the formulae given by the adiabatic theory, and proposes 

 other formulae in their stead. Finally, the secondary phe- 

 nomena in the jet have been investigated by E. Mach and 

 Salcher, L. Mach, R. Emden, and others §. 



Theory. 



Assuming (1) That viscosity effects are eliminated, 



(2) That the ideal gas laws hold, 



(3) That the outflow is adiabatic, 



(4) That the motion is steady, 



the theory given by Saint- Venant and Wantzel, and extended 

 by Hugoniot and Osborne Reynolds, leads to the formula : — 



Ma S , effl u X =s^(g>v^ii[^e-:F]. 



where S / = area of vena contracta, 



p = pressure inside gas-reservoir, 



j»i = external pressure, 



7 = ratio of two principal specific heats, 



T = temperature of quiescent gas in reservoir, 

 provided the external pressure is greater than a certain fraction 

 of the internal pressure in the gas-reservoir. 



I have not carried out experiments under the conditions 

 required by this formula for several reasons: — (1) The 

 formula does not lead to simple comparative results for two 

 gases possessing different values for y. (2) The efflux in this 

 case is slower, and therefore less likely to be adiabatic. (3) 

 The efflux is dependent on the back pressure, which introduces 

 an additional variable factor in comparative experiments, if, 



* C. JR. 1886, passim ; Ann. de Chim. et de Phys. Nov. 1886. 



t Phil. Mag., March 1886. ; 



% Ann. de^Chim.et de P/w.s. viii. pp. 1-79 (1896), xii. pp. 289-37S 

 (1897). * v 



§ Wied. Ann. lxix. 1, pp. 264-289 (1899), where Emden gives re f er 

 ences to the previous literature. 



