61 Prof. W. A. Norton on the Physical Constitution of the Sun. 



envelope is composed that may chance to be present is urged up- 

 ward by a force of repulsion. If, as we must suppose, the rise 

 of the solar vapours from the surface A continued for an indefi- 

 nite time, the interpenetration of contiguous envelopes would 

 increase, and eventually a condition of equilibrium would be 

 attained, if the sun's temperature remained the same. But if 

 this temperature were to increase, as it must down to a certain 

 epoch in the process of consolidation, the process above indi- 

 cated would be continually renewed. It is still more important 

 to observe that if there were any cause in operation withdraw- 

 ing continually at short intervals a portion of one or more of 

 these rising vapours, a statical equilibrium would not be reached ; 

 and it would be permanently true that /or every such vapour there 

 would be a region of repulsion, as above stated, extending from its 

 envelope dGivn to the outer limit A of the vapour of greatest atomic 

 weight. Throughout this region the vapour would be perpetu- 

 ally rising, taking the place of that which is withdrawn, and so 

 maintaining a dynamical equilibrium. The depth of this region 

 would be the greatest for hydrogen, the outermost gas (unless 

 there is some solar vapour of less atomic weight than hydrogen). 

 Now it is easy to see that a certain physical cause tending to 

 produce such results must come into operation at some stage 

 of the sun's process of consolidation. As conceived by Faye, 

 the cooling going on at the outer surface must eventually bring 

 the temperature there down to the point at which the vapours 

 having the highest affinity for oxygen will undergo combus- 

 tion. The product of such combustion, being compound mole- 

 cules, will have a greater weight in comparison with the repul- 

 sion to which they are exposed than the simple molecules 

 before the combination took place, and hence they will descend 

 more or less rapidly into the depths of the photosphere. To 

 all appearance the sun is now passing through this period of its 

 physical history, as supposed byFaye; and in the "granula- 

 tions" which give to the solar disk a mottled appearance 

 (HerschePs "subsiding chemical precipitates") we probably 

 discern the products of the combustion occurring in the upper 

 photosphere and determining its outer limit in the act of de- 

 scending. The continual upward flow, from the depths of the 

 photosphere, of the hydrogen, oxygen, and the lighter metallic 

 vapours will bring about the necessary intermixture of oxygen 

 with the other vapours. This must occur below the natural 

 outer limit of the hydrogen envelope ; and we know that, as a 

 matter of fact, the chromosphere, composed chiefly of hydrogen, 

 extends above the photosphere. 



If the products of the surface-combustion were all to descend 

 indefinitely into the vaporous photosphere without undergoing 



