460 Mr. J. Evershed's Experiments on 



and thus F can never assume negative values. Also 



F=M2a 2 + 2& 2 + 2 C 2 -|0 2 + A 2 + B 2 + C 2 ), . . (29) 



this is the expression for the " Dissipation Function " F 

 originally given by Lord Rayleigb. The cause for this 

 denomination is obvious. Suppose a, b, c, A, B, C to have 

 their sign changed ; the sign of pQ will change but that of 

 F will not ; hence the term pd corresponds to a reversible, 

 the term F to an irreversible phenomenon. Indeed the 

 conversion of molar energy into molecular energy which 

 o-oes on, of whatever kind the existing disturbance may be, 

 fs irreversible. We have thus before us an example of 

 Dissipation of Energy in a purely dynamical system. 



3. If we now suppose that every molecule possesses an 

 amount MA of internal energy in addition to that which 

 depends on its motion of translation, we shall have 



l[tp(tf + tfi + v?+?W+P)+fi'i = Q- . (30) 

 ot 



Now it is easily seen that Bh/Bt and dh/dt are equivalent, since 

 h cannot be affected by external forces any more than by con- 

 vection ; therefore 



lp | (f * + v ' + ? T + p § + pf< i + pv'l' + p¥o + pyt, A + pKk B + pTvC 



+ i*(pW 2 + pW+ P ?) = 0. ...,. (31) 



Maxwell puts ph = i(/3— l)p(f 2 + ?? 2 + f 2 ); if this relation is 

 adopted, equation (31). will be that numbered (94) in Max- 

 well's paper, with a difference of factor, however, in the terms 

 relating to conduction of heat. The foregoing deduction 

 will not be subject to the criticism which M. Poincare has 

 offered with respect to Maxwell's deduction (see Comptes 

 Rendus, vol. cxvi. p. 1017 ; see also ibidem, p. 1165, where 

 the internal energy of the molecules is taken into account in 

 the same way as that here adopted). 



XLVL Experiments on the Radiation of Heated Gases. 

 By John Evershed*. 

 [Plate VII] 



THE renewed interest aroused in certain fundamental 

 questions in spectroscopy by the publication of the 

 researches of Pringsheim, Paschen, and Smithells has led me 

 recently to undertake a series of observations on the visible 

 * Communicated by the Author, 



