212 Dr.. Meyer Wildermah: Connexion between the 



Now we carry out the following isothermal process : — 

 1. We remove the light from the same : the Gl 2 will com- 

 bine with the Ag 2 (or Ag 2 Cl) giving 2 gr. mol. AgCl. 

 During this process of combination the work done by the 

 piston against the system will be pv = WI; Apv = very nearly 

 2 cal., which will be given to the system ; the heat of the 

 reaction of the combination in the dark will be W d , and this 

 will be taken from the system. 



. 2. Now we expose the system to light. It will take up 

 energy from the light, and the light-kinetic energy used up 

 during the reaction will be Ej ; if L be the light-kinetic 

 equivalent of heat, LE^ is given to the system during the 

 reaction in light. During this reaction 1 gr. mol. Cl 2 is 

 formed, which keeps Cl 2 , AgCl, Ag 2 (or Ag 2 Cl) in equilibrium; 

 the work done by the system is —pv = — RT; — Apv = —2 cal., 

 and the heat of reaction of dissociation in light is Wi and 

 this is given to the system. The system is thus again in the same 

 state from which we started, therefore — W d -+- W z + LE ? = 0, 

 i. e., the hpat of the same reaction is at the same temperature in 

 light not the same as in the dark. If W^ is negative, Wj is 

 < W<z; if Wd is positive, Wi is>Wj. Now in the equation 

 dFi — tdn—pdi\ tdi) is the heat taken up by the system 

 (positive or negative) when it is passing from one state to 

 another. Therefore at the same temperature t in the light tdrf 

 or Wj is different from tdrj or W^ in the dark. 



Further, the exposure of the system to light cannot very 

 often remain without a change in the mechanical energy pdv 

 of the system, namely, when chemical transformations take 

 place in the same. It is, however, not impossible that this 

 is also the case even when no chemical transformation takes 

 place in the system, however small this change in the value 

 of pdv may be, considering that the pressure of a gas at a 

 constant volume ought to change with the variation of the 

 kinetic energy of the atoms and molecules. Thus our previous 

 equation for equilibrium of a system with independent vari- 

 ables assumes when it is exposed to the light or to the action 

 of electric waves, and when the heat produced by the absorbed 

 light is removed from the system (say by the surrounding- 

 bath of a constant temperature t), the following form : — 



dE + dE^dW 



= t'dr/-p>dc'-t J (fM l f dm 1 ')+ j (fi/dm 2 ')... + i i 1 !dm n f ) ft . 



where E' is the total energy of the system in light, rj 1 its 

 entropy, v the volume, t'r)' the thermal energy, p'v' the 

 mechanical energy in light, (miW)j (m 2 W)- • .(/*»W) is its 

 chemical energy, and (X/m/), (X 2 V)...(\/w„ / ) the new 



