of the newly discovered gases of the Earth's Atmosphere. 91 



such velocity from that surface that it will not return, provided 

 it be actuated only by the kinetic energy of the gas and by 

 gravitation. Dr Bryan is careful to point out that other con- 

 siderations may entirely alter the problem. Now the postulate 

 of a definite surface to the earth's atmosphere and no gas to 

 be encountered outside it cannot be conceded, nor can the motion 

 of the earth in its orbit be left out of account. If we talk of 

 a gas leaving the atmosphere we need define the limit of the 

 atmosphere, and that is not easy. I do not think that it has any 

 definite limit, but we may make an arbitrary one. Shall we say 



that when the density is reduced to -^— of what it is at the earth's 



surface we have reached that limit ? At all events I do not think 

 there is any evidence that the density of the residuum of gas in 

 planetary space is less than that. If we take Dr Bryan's figure (de- 

 rived I think from Lord Kelvin) that the number of molecules in 

 one cubic centimeter of gas at normal pressure and temperature is 



approximately 10 21 , and suppose it reduced to y^ of that, we 



shall still have 10 9 molecules left in each cubic centimeter, and 

 a molecule running into such a crowd, has as good a chance of 

 being knocked further away as it has of getting back. In fact 

 if we consider a layer of air at this density, one cubic centimeter 

 thick, all round the earth, and remember that the free path is 

 very long, even if we suppose that the molecules all have only the 

 average kinetic energy due to their temperature, many of them will, 

 in a small fraction of a second, have passed far into the layers 

 outside, and it is an equal chance whether the same molecules get 

 back, or others are knocked back instead of them. We do not know 

 at all what the temperature of the residuum of gas in planetary 

 space may be, but it is probably very different from that which 

 a black body would assume if left there. The diffusion of each 

 layer into those above and below it will jgo on, even at a low 

 temperature, with rapidity ; and though it is true that the layer 

 will contain but an infinitesimal fraction of the earth's atmosphere, 

 yet as it is diffused into neighbouring layers in a small fraction 

 of a second, and there are more than 30 millions of seconds 

 in a year, and the solar system has been many millions of years 

 in settling into its present state, there has been time enough 

 for some considerable interchange of the atmospheres of the sun 

 and earth. It is, however, far more probable that the solar system 

 has settled into its present state from a nearly uniform gaseous 

 mass, than that the materials out of which the sun and planets 

 have been aggregated should have been distributed in such a 

 heterogeneous way as to give a marked distinction between the 

 atmospheres. Nor can we suppose that the sun and planets can 



