Energy of Light and Chemical Energy. 



211 



appears, but the plate in the light always gradually assumes 

 {passing through an induction period) constant new properties ; 

 a constant new electromotive force is created, which is to be 

 seen on the photographed curve, from a line parallel to the line 

 in the dark. Dozens o£ photographed curves obtained with 

 different metals establish this generalization. (2) The electro- 

 motive force is directly proportional to the intensity of light 

 and is a function of its composition. 



Thus in the above equation (12) new terms \ x dm u \ 2 dm 2 ... 

 \ n dm n will be added to indicate the new kinetic energy stored 

 by light in the different components of the system. 



Further reflection leads to the conclusion that this variation 

 in the energy o£ the system under the action of light cannot 

 remain without effect upon the entropy of the system. In 

 the general equation (12) tdw is the thermal energy of the 

 system, and since the same consists not only of molecular but 

 also of atomic motion, it cannot, therefore, remain the same 

 when the atomic motion in the different molecules of the 

 system changes under the influence of light. We are also 

 able to give evidence for this a priori conception ; we shall 

 carry out for this a cyclic process at a constant temperature, 

 making use of the following system: Ag 2 (or Ag 2 Cl)-r-01 2 

 = 2AgCl. It is known that Ag 2 (or Ag 2 Cl) and Cl 2 com- 

 bine in the dark completely to AgOl. It is also known that 

 AgCl decomposes under the action of light either into Ag 2 

 and Cl 2 or into Ag 2 Cl and CI, (this point is undecided yet) ; 

 and consequently if AgCl is exposed to light in a closed 

 vessel, this decomposition of AgUl will go on for so long 

 until just as much AgCl is formed in the unit of time 

 from Ag 2 (or Ag 2 Cl) and Cl 2 , as AgCl decomposes under the 

 action of light into Ag 2 (or Ag 2 Cl) and Cl 2 ; the system will 

 then be in a state of equilibrium, i. e., 



Ag 2 (Ag 2 Cl) + Cl s = 2AgCL 



Let us now assume we have 

 at the temperature t the svstem Fig. 2. 



Ag 2 (or ClAg 2 )+Cl 2 =2AgCl, 

 exposed to light, in equilibrium. 

 The same is in a cylinder with a 

 piston, which moves without 

 friction, and exerts a pressure 

 upon the gas Cl 2 contained in 

 the same of one atmosphere, the 

 A&\, or AffCl or Ao Cl and 

 AgCl being solid. Let the quan- 

 tity of CJ 2 be =1 gr. mplec; it 

 will occupy the space of 22 litres 



1*2 



I ATT. t' 



Ag.Cl+AgOl 

 or 

 Affo+AoCL 



