510 Dr. Gr. Johnstone Stoney on Escape 



from the atmosphere by the molecules gaining intermolecular 

 energy from the aether, to a much greater extent than 

 molecules do under normal conditions of temperature and 

 pressure, which intermolecular energy is transformed into 

 energy of motion during collision, or whether, as I would be 

 more inclined to believe from the results of my own investiga- 

 tions and those of Dr. Bryan, the earth is able to retain its 

 helium, is still in my mind a question for further more careful 

 investigation. The Maxwell-Boltzmann equation should be 

 made to include all the variables discussed by Dr. Stoney, 

 which will in any way have an influence on the speed of the 

 molecules, under the most attenuated conditions of the gas, 

 and by the application of this more comprehensive equation 

 to an atmosphere of helium at the possible ultra-atmospheric 

 temperature, a much more probable result would be reached. 



I am sure that no one will be more ready to accept the 

 results of such an investigation than the writer, and with the 

 sincere hope that the present discussion may induce some of 

 the mathematicians to enter this field of scientific inquiry, 

 I am, dear Sirs, most faithfully yours, 



Cornell University, S. R. COOK. 



Ithaca, New York, U.S.A.. 



XLVIIL Dr. G-. Johnstone Stoney's Reply. 



Thkough the courtesy of the Editors of the Philosophical 

 Magazine I am given the opportunity o£ replying to Mr. Cook's 

 letter in the same number of the Magazine in which his 

 letter appears. 



Mr. Cook in this letter repeats his belief that he had 

 " attacked the problem from the standpoint of Maxwell's 

 equation for the distribution of velocities in a gas." This 

 seems to overlook the material circumstance that no such 

 equation exists. On the contrary, Maxwell is careful to point 

 out again and again that his law only gives the distribution 

 of velocities in an artificial kinetic system consisting of hard 

 elastic particles of equal mass, and in this model only " after 

 a great number of collisions " have taken place. He shows 

 that the law for viscosity within such a kinetic system and 

 also the law for the diffusion of one such artificial system 

 into another, are nearly the same as the corresponding laws 

 for at least some of the gases of nature, when they are under 

 the conditions which prevail at the bottom of our atmosphere. 

 He was also the first to show that these conditions are such 

 that the number of encounters met with by each molecule of 

 the air at the bottom of our atmosphere is about eight 



