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XXXVIII. 



WHY THEEE IS NO ATMOSPHEEE ON THE MOON, 

 By G. JOHNSTONE STONEY, F.E.S. 



(Abstract of a Fajoer read April 20, 1892, and published in extenso in the Scientific- 

 Transactions OF THE Royal Dublin Society, Vol. IV., Part xiv., p. 703.) 



In this communication reference is made to the conditions 

 that limit the height of an atmosphere upon any celestial body. 

 These had been announced by the author in a Paper " On 

 the Physical Constitution of the Sun and Stars," printed in the 

 Proceedings of the Royal Society for 1868 ; and in the present 

 Paper it is pointed out that the same method of investiga- 

 tion shows that under certain circumstances some of the con- 

 stituents of an atmosphere may molecule by molecule wander ofE 

 into space. This event occurs with more readiness — 1st, the 

 lower the mass of the molecules of the gas ; 2nd, the higher 

 the temperature at the boundary of the atmosphere ; 3rd, the 

 feebler the potential of gravity at the boundary of the atmo- 

 sphere. 



By investigating the conditions that prevail on the earth 

 and moon, it is shown that free hydrogen could not remain a 

 constituent of the earth's atmosphere ; and that no free oxygen, 

 nitrogen, or the vapour of water could remain on the moon. 

 Hence, even if there were no oxygen present, the earth's atmo- 

 sphere could not retain free hydrogen ; and on the moon there is 

 now neither atmosphere such as we know it, nor water nor ice. 



It follows from the investigation that space must be peopled 

 with vast numbers of wandering gaseous molecules, especially of 

 the lighter gases ; and that these tend ultimately to settle down 

 upon such of the more massive bodies of the universe as are 

 sufficiently dense to exert a powerful potential at their surface. 



Finally, the investigation indicates conditions which must be 

 fulfilled by any "nebular hypothesis" in order that it may be 

 admissible. 



