On Inrjuirie* as to the Escape of Gases from Atmospheres. 289 



vessel, and at each such encounter there is a partition of energy 

 between the translational and the internal motions, and in consequence 

 of this the kinetic energy of the translational motion becomes a part of 

 what determines that average power of absorbing and emitting radiant 

 heat which (when estimated over a time embracing a sufficient number 

 of encounters) is the proper definition of the radiation temperature of 

 the molecule. Accordingly the average kinetic energy of the transla- 

 tional motions of the molecule enters into its mathematical expression. 

 If the gas be dense, encounters are frequent, and A, the time requi- 

 site for the averages, may be brief. In this case the radiation tempera- 

 ture of a molecule, while the gas is undergoing some change in its 

 condition, is predominantly the oiitcome of its encounters, and depends 

 mainly on the molecules that surround it ; whereas if the gas be very 

 much attenuated, then the radiation temperature of the molecule 

 during a period of transition will depend mainly on what influences 

 then reach it from the surrounding aether, and will be but in a subordi- 

 nate degree affected by the encounters to which the moleciile at aboiit 

 that time happens to be subjected. 



This is a matter which needs to be very fully taken into account 

 when we attempt to estimate the escape of molecules from the earth's 

 atmosphere, inasmuch as a large part of the heat radiated by the sun 

 to the earth is absorbed by the gaseous molecules which happen at 

 the time to be moving about in those strata of the atmosphere from 

 which alone there can be any effective escape. Accordingly it will 

 .need to be carefully scrutinised whether this has been either explicitly 

 or implicitly taken into account in the attempts which have been made 

 to determine a priori the rate of escape. 



W4ien the molecules of a gas or of a mixture of gases move in a 

 field of force such as that surrounding the earth, convection currents 

 can exist, and the term temperature as applied to the gas becomes 

 ambiguous. It may have either of two distinct meanings, one of which 

 has reference to the transport of heat by convection and by the con- 

 sequent sweeping of successive portions of gas against bodies immersed 

 in it, and the other has reference to the exchanges of heat by radia- 

 tion with those or with more distant bodies. These are different 

 physical events, and the assumption that they stand in a fixed ratio to 

 one another is convenient, but is often not true. It is probably legiti- 

 mate to regard it as approximately holding good in a gas which has 

 nearly reached a final, i.e., an unchanging condition, and where the 

 problem with which we are dealing does not need our making any 

 closer scrutiny than as to what on the average happens to a sufficiently 

 large swarm of molecules within a sufficiently long duration ; but it is 

 not true while gas is passing through transition stages, nor is it true of 

 individual molecular events or of small swarms of events, even in gas 

 which has reached its final state. 



VOL. LXVII. Y 



