512 Escape of Gases from Atmospheres. 



is mistaken. I was certainly most desirous o£ representing his 

 work correctly ; and in fact the numbers he gave and again 

 gives in the present letter, are, as I have already said, suffi- 

 ciently in agreement with those I had worked out. It is not 

 here that we differ; but in the inference we draw from those 

 numbers. Mr. Cook does not think them inconsistent with 

 the fact that the Moon's atmosphere has escaped, while I do. 

 He finds that a temperature of 270° C. above freezing-point, 

 or of 543 absolute, was requisite for the escape of carbon 

 dioxide. But he does not refer to the further fact that no 

 such temperature can have prevailed in the lunar atmosphere 

 when it had one. [In this discussion the temperature of a 

 gas is of course regarded from the usual limited point of 

 view, which represents it as proportional to the kinetic energy 

 of the translational motions of the molecules.] 



The best determination which we seem to possess of the tem- 

 perature of a small fully absorbing body at the distance from the 

 Sun of the Earth and Moon seems to be Professor Poynting's 

 (see Phil. Trans, vol. 202. (1904) p. 535) . It is 300° Absolute, 

 which is the same temperature as 27° G. above freezing-point. 

 Again, for the upper limit which could be reached by the 

 temperature of the hottest part of the surface of an airless 

 Moon, he obtains 412° Absolute, which is the same as 

 137° C. 



Accordingly the highest temperature at our command 

 seems to lie between 27° C. and 137° C, and probably nearer 

 the former temperature than the latter. It thus appears that 

 the 270° C. which Mr. Cook requires is far beyond what we 

 are justified in regarding as possible. Langley, who has 

 also investigated lunar temperatures, assigns still lower tem- 

 peratures than Poynting's. 



I think, therefore, that the numerical results which 

 Mr. Cook obtains from misunderstanding Maxwell's formula 

 do not justify his statement that they " explain fully the 

 escape of the atmosphere from the Moon." The correct 

 inference from Mr. Cook's results seems to be precisely the 

 opposite, and supports the inference which I drew that 

 Maxwell's formula is misapplied if employed upon this 

 problem. 



Portunately, the inductive method is also available for 

 investigating the rate at which gases actually do escape from 

 atmospheres. And, still more fortunately, it is free from the 

 pitfalls which beset the steps of the mathematician who 

 attempts to employ the deductive method upon a physical 

 problem of this kind, unless he is very careful to keep in 

 miad that his argument consists of deductions from assumed 



