i) 



[ 508 ] 



XLVII. Escape of Gases from Atmospheres. 



To tlie Editors of the Philosophical Magazine. 

 Gentlemen, 



A LETTER under the above heading, by Dr. G. Johnstone 

 Stoney, in reply to my note in ( Nature ' of the 24th 

 of March, published in the June number of the Philosophical 

 Magazine, has just come to my notice, and I am sorry to 

 observe that it places some of the results of my former 

 researches on the above subject in a false light. I therefore 

 beg the hospitality of the Philosophical Magazine for a very 

 brief note. 



In the investigations on the escape of gases from atmo- 

 spheres by Dr. Stoney, Dr. Bryan, and the writer, two widely 

 different conclusions have been reached by two distinctly 

 different methods. Dr. Stoney takes the position that helium 

 is escaping into the atmosphere through springs and other 

 natural sources, at a greater rate than it is by any means 

 whatever being returned to the earth ; and, since it is 

 chemically a very inert gas, it is not uniting with any 

 atmospheric gas, and hence the only way to account for the 

 removal of so large a portion of the helium from the 

 atmosphere is by assuming that it escapes into outer space*. 

 The very great importance of this assumption, with its many 

 and very interesting applications to the kinetics of atmospheres, 

 led me to attempt to verify this assumption by applying the 

 laws of the kinetic theory to the escape of molecules from 

 the atmosphere. After giving the most liberal interpreta- 

 tion to the laws, and after taking into consideration every- 

 thing that could consistently be employed, in the light of our 

 present knowledge of the kinetic theory, that would assist 

 the escape of the molecules in the highly attenuated atmo- 

 sphere, it was found that only 13 X 10 8 c.c. of hydrogen and 

 only 10 X 10~ u c.c. of helium would escape in a year. 



The real point at issue between Dr. Stoney and the writer 

 is the validity of the method of attacking the problem. 

 Dr. Stoney's first memoirs on the subject appeared before 

 helium had been shown to be a measurable constituent of the 

 atmosphere ; and the fact that helium had been poured 

 into the atmosphere for ages and was not found in the 

 atmosphere, formed the basis of Dr. Stoney's theory that 

 helium was escaping from the outer limits of the atmosphere. 



* " On Atmospheres upon Planets and Satellites," Trans, of the Royal 

 Dublin Society, vol. vi. p. 305 (Oct. 1897) ; Astrophysical Journal, 

 vol. vii. p. 25 (January 1898). 



