Escape of Gases from Atmospheres. 509 



The writer* attacked the problem from the standpoint of 

 Maxwell's equation for the distribution of velocities in a gas, 

 and showed that, it' the law of the distribution of velocities 

 holds for a gas at normal temperature and pressure, helium 

 would not escape from an atmosphere of helium whose outer 

 limits were at 5° 0. and in other conditions similar to the 

 earth's atmosphere at normal pressure, with nothing but 

 gravitation to prevent the molecules from flying off into 

 space. Dr. Stoney criticises the use of Maxwell's formula for 

 the distribution of velocities, as applied to the highly attenuated 

 gas in the limits of the atmosphere f. He maintains that 

 there are causes, other than those comprehended within the 

 Maxwell or the Boltzmann-Maxwell law, which would cause 

 the molecules in the outer atmosphere to assume velocities 

 much greater than under normal conditions. So that if it 

 is Maxwell's velocity function, and S a function made up of 

 all other factors that would cause the molecules to attain to 

 a critical velocity : then the total number of molecules that 

 would attain to the critical velocity would be 



h(tt+S). 



Allowing the validity of this equation, I have shown J, that 

 even though 8 may attain a value of 100 7r, the permanency 

 of an atmosphere of helium on the earth would not be 

 materially affected. 



One point that I wish especially to call attention to in Dr. 

 Stoney's letter, which puts my data in a false light, is on 

 page (591, where he says : u What convinced me several decades 

 ago that the conclusion at which I arrived, and at which 

 Mr. Cook has arrived, is false, is that it represents the moon 

 as incompetent to get rid of the atmosphere which it originally 

 shared with the earth, and ofthegases which ithas since evolved 

 in abundance from its own interior." In answer to this I only 

 need to cite the results of my investigation, which explain 

 fully the escape of the atmosphere from the moon. Quoting 

 from my first paper (Astrophvsical Journal, Jan. 1900, 

 p. 41) : "This table (a table showing the temperature which 

 the moon, the earth, and three of the planets, would necessarily 

 attain in order that an atmosphere of air should escape) 

 .-hows that an atmosphere of hydrogen would escape from 

 the moon with its outer layer at — 256° C, an atmosphere of 

 air at — 10° C, and an atmosphere of carbon dioxide at 270° G." 



Whether, as Dr. Stoney believes, helium is really escaping 



* Astrophysical Journal, vol. xi. p. 36 (January 11)00). 

 t Astrophvsical Journal, vol. xi. pp. 25] & 3^8 (1000). 

 t '-The Permanency of Planetary Atmospheres/" Monthly Weather 

 Ite\iew. August 1002. 



