776 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



the before-mentioned principle of mobile equilibrium, which he for- 

 mulates thus : "All equilibrium between two different states of matter 

 is displaced in consequence of the lowering of temperature toward 

 the system the formation of which develops heat." It is known 

 that this principle a little later was generalized by Le Chatelier, 

 who extended it not only to thermic variations, but to all altera- 

 tions of external and internal conditions of equilibrium (pressure, 

 electric state, concentration, etc.). He writes, therefore, thus: 

 " Every external action impresses on a body or on a system a change, 

 the direction of which is such that the resistance offered by the body 

 or by the system to the external action is increased." This principle 

 preserves its importance even beside the second principle of thermo- 

 dynamics, for although it authorizes onl}^ qualitative conclusions, it 

 may no less be used with success when it is a question of determining 

 the direction in which a certain process will take place, and especially 

 in the cases where it is not possible to bring the problem to the 

 mathematical form of the equation of Clapeyron or to other analo- 

 gous formulations of the second principle. It can at the same time, 

 thanks to its simple and general form, be easily remembered, since it 

 implies the idea of a sort of faculty of accommodation to exterior 

 actions inherent in matter. 



Van't Hoff announced this principle, as we have said before, only 

 for thermal variations, but to attempt, for this reason, to detract 

 from his merit, as certain authors do, appears to us profoundly un- 

 just, for not to mention his indisputable priority, one must not forget 

 that the case treated by him is much the most important for chem- 

 istry. He succeeds thus in settling a question which had been engag- 

 ing chemists for a long time, since the problem concerning the direc- 

 tion toward which a recreation moves is, it will be admitted, of 

 fundamental importance. 



It is known that Thomsen had, several years before, announced a 

 rule oh which Berthelot wanted later to confer the dignity of a natu- 

 ral law — the principle of maximum work, according to which, of 

 all possible reactions, those might be produced spontaneously and 

 without intervention of foreign energies, the production of which 

 is accompanied by the greatest development of heat. 



Now this rule, which is verified in practice for most of the ordi- 

 nary reactions of chemistry, is found at fault in different well-stud- 

 ied cases of chemical reactions strictly speaking and in entire series 

 of processes, like the reversible processes. ^V\u\e Thomsen recog- 

 nized the empirical character of his rule, Berthelot sought on the con- 

 trary to save his principle by the aid of a series of ingenious reason- 

 ings based on the ambig-uity which is attached to the use of vague 

 expressions, such as " to be produced spontaneously " and " without 

 the intervention of external energy." But it was labor lost, so that 



