Energy of Light and Chemical Energy. 217 



reverse phenomenon to the hist, i. e-., during this period the 

 kinetic energy acquired under the influence of light gradually 

 falls from its maximum to zero. Experimental evidence 

 'for the deduction period is given in the experimental in- 

 vestigation of the velocity of combination of CO and Cl 2 *. 

 . As in the case of the induction period, here we must also 

 distinguish between the two periods — the " chemical " de- 

 duction period at the beginning when light is removed, usually 

 lasting only a short time (the reaction stops), and the much 

 longer " deduction period of energy/' Though chemical 

 reaction very soon stops in the system, it is still in another 

 state of energy, and it only gradually returns to its previous 

 •state of energy in the dark before the illumination. This 

 was shown by the author on the length of the induction 

 periods on fresh exposures of the system CO + Cl 2 to light. 

 As in the case of the induction period, both the acquired 

 new chemical and the light-kinetic energy decrease simul- 

 taneously during the two periods of deduction. It may be, 

 however, that the ratio of the newly-acquired chemical 

 energy to that of the newly-acquired light- kinetic energy 

 still remaining in the system on removal of light, is not 

 always the same during the period of deduction. The 

 induction and the deduction period evidently equally concern 

 systems in which chemical reaction goes on both in the light 

 and in the dark, but with different speeds ^H 2 2 ), or systems 

 in which a reaction goes on in light only (CO + Cl 2 or 

 H 2 + C1 2 ). It further follows that if alter the deduction 

 period is completed the system should again be exposed to 

 the light, the system will evidently have to pass again through 

 a period of induction ; and if the light should be again 

 removed from the system, a new deduction period will have 

 to follow. Also, that during the induction period, i. e., 

 before the constant maximum of kinetic energy is readied, 

 the longer the system was previously exposed to the induc- 

 tion, the longer will be the deduction period through which 

 the system passes to the old state, and that, after the 

 induction period has passed and the maximum reached, the 

 deduction period will always last the same time, however 

 long the system may have been exposed to the light ; so 

 also the less the system reached the old state, i\ e., the less 

 the deduction period is completed, the shorter is the induction 

 period when it is again exposed to the light : it is also 

 evident that both the induction and deduction periods must 

 naturally depend upon the chemical composition of the 

 system at the different times, and upon the intensity and 



* Pllil. Trans. 199 A, p. 337. 



