395 
AND STATICS UNDER THE INFLUENCE OF LIGHT. 
or of chlorine and hydrogen, are to be measured quantitatively. The author is at 
present engaged in such attempts to test directly the constants of equilibrium, not so 
much because the law needs further confirmation, as on account of the very interesting 
thermodynamic connection which must exist between the constant of equilibrium, the 
heat of reaction in light, and the absolute temperature on the one hand, and the 
constant of equilibrium and the intensity of light on the other. 
Appendix : Thermodynamical Considerations . 
The above experimental results find their rational basis and explanation in thermo¬ 
dynamics. The condition of equilibrium in a homogeneous system, when oidy 
chemical, thermal, and mechanical energy are taken into consideration, is according 
to Gibbs: c/E = t dy — pdv + y l dm 1 + p, 2 c?m 3 . . . + \i, l dm n 0, where E is the 
energy, y the entropy, m l5 m. 2 . . . m n the quantities of the substances S l5 S 2 . . ., 
the chemical potentials of S l5 S 3 . . . Let us now assume that the system is exposed 
to light of constant intensity and composition, and that the system is in such thin 
layer that the intensity of the light is the same in all parts of it. Since all 
substances absorb light and the light absorbed is not completely transformed into 
heat, a part of the light will appear as other forms of kinetic energy of the atoms 
and molecules. From a molecular mechanical point of view this will mean that 
under the influence of light the amount of work present in the molecules as energy 
of the atoms increases. Obviously the ratio of the amount of light transformed into 
heat to that transformed into kinetic energy of the atoms is not constant. At first 
the energy of the atoms and molecules gradually increases (induction period of 
energy), until a reaction, a shifting of the point of equilibrium to another one, becomes 
possible (chemical induction period), which is observed by an increase of the velocity 
constant. Under the action of light the storage of energy in the atoms and molecules 
ultimately reaches a maximum, after which light produces no more strain upon the 
atoms, preventing them only from losing the energy once acquired, and the whole of 
the light entering into the system is transformed into heat. This maximum kinetic 
energy of the atoms is a function of the intensity and composition of light, of the 
nature of the substance, of the surrounding medium, &c., and becomes apparent in 
the fact that a velocity constant, indicative of constant properties of the atoms and 
molecules, is obtained. When light is removed from the system the energy stored in 
the atoms and molecules, under the impulses of the light waves, gradually disappears, 
changing either into chemical energy and heat (as long as the reaction continues in 
the dark, chemical deduction period) or into heat alone (deduction period of energy). 
When the maximum kinetic energy of the atoms is reached under the action of light, 
it is evident that the energy stored up must be directly proportional to the mass of 
each substance. To the above equation for equilibrium the terms v l dm l + y,cZm 2 . . . 
+ v, l dm )l must therefore be added. By means of a cycle process at a constant 
