Reaction, before Complete Equilibrium. 55 



light or to electric waves \D?n, its magnetic energy vDm. 

 Then the total energy of the mass is 



jDe -f §ghDm -f §\Dm + JvDm. 



The condition of equilibrium will be that 



SJDe + 8J\<,hT>m + 8 j\Dm + 8 jVDw >0. 



Let Dr? and Dr be the entropy and volume of the element 

 Dm. Then we have according to (12), 



8De = t8I>7j — p8Dr + /jL l 8Dm l + /jL 2 8Dm 2 • • • 4-/*«SDm». 

 The condition of equilibrium will be 

 8 JDe + SjV*Dm + S$\Dm + 8JVD w 

 rsf^Dr/ — \p8Dv + J//, 1 SDm 1 +J / u, 2 SDm 2 . . .. +J/&»SD»i„ 



+ \\ 1 8Dm 1 + ("XoSD/^o • • • +fA.„SD»j n 

 +]v l 8Dm l +J v 2 8Dm 2 . . . +jV tt 8Dm«>0. 



Now the different parts o£ the general condition of equilibrium 

 are dependent upon separate equations of condition, so that 



§t8V V must be >0, when j*8Dr7 = ; ... (a) 



^c/8hl)nt — \j/8Dr must be ^0, when the envelope containing 



the system is assumed to be fixed and rigid : . . (/3) 



j fi l 8Bm l + J(//*SDra, +^X 1 8Dm l +§v 1 8Dm l 



must be >0, when J 8Dm l = 0, 

 \/J, 2 8Dm. 2 +\ ; //,8Dm.,+§\ 2 8D)n 2 +^v 2 8Dm 2 



mst be >0, when §8Dm 2 =sO, 



nn 



must be >0, when §8Dm„ = 0. . . (7) 



Equation («) gives the condition of thermal equilibrium 

 under the conditions mentioned here : inconstant, the tem- 

 perature must be the same through the whole system. 



Equation [ft) gives the condition of mechanical equilibrium 

 when the system' is under the influence of gravity. It can 



