244 



SCIENCE. 



[N. S. Vol. XI. No. 268 



self, in the lecture which he was invited to 

 deliver before the German Chemical Society 

 in 1894. As we have seen, he was early 

 occupied with the study of the position of 

 atoms in space. From this he was led to 

 study the velocitj' of reactions and the con- 

 ditions of equilibrium in chemical processes. 

 But the problem of aflSnity was closely con- 

 nected with that of equilibrium. As an 

 example of affinity he studied the attrac- 

 tion of salts for their water of crystalliza- 

 tion. He found this to be very small and 

 adds: " I had the impression that even the 

 weakest chemical forces are very large. 

 * * * The question arose whether it is not 

 possible, in simpler cases, to measure the 

 attraction for water more directly ; and for 

 this purpose the aqueous solution is the 

 simplest conceivable — -much simpler than 

 the compound containing water of crystalli- 

 zation. Coming from the laboratory with this 

 question in mind, I meet my colleague 

 De Yries, and his wife. He was just at 

 that time carrying out osmotic investiga- 

 tions, and he told me about Pfeffer's deter- 

 minations." 



Van't Hoff was thus introduced to the 

 work of Pfeffer, in which the latter had 

 measured the osmotic pressure exerted by 

 aqueous solutions of a number of substances, 

 when separated from the pure solvent by a 

 semipermeable membrane, through which 

 the solvent, but not the dissolved substance, 

 could pass. Van't Hoff observed from 

 Pfeffer's results, that the osmotic pressure 

 exerted by any substance at a constant tem- 

 perature is proportional to the concentra- 

 tion of the solution, and saw in this an 

 analogy to Boyle's law for gas-pressure. 

 The gas-pressure of a gas is proportional to 

 the concentration of the gas. Having found 

 this one relation between osmotic pressure 

 and gas-pressare, he tested other laws of 

 gas-pressure by the osmotic pressure of so- 

 lutions, and found that the law of Gay 

 Lussac for the temperature coefiicient of 



gas-pressure, applies also to the temperature 

 coefficient of osmotic pressure. These two 

 laws of gas-pressure might apply to the os- 

 motic pressure of solutions, and still the ab- 

 solute value of the two pressures be very 

 different. 



The fundamental question still remains : 

 Is there any close relation between the 

 actual pressure exerted by a gas, and the 

 osmotic pressure exerted by a solution con- 

 taining the same number of dissolved par- 

 ticles in a given space as there are gas par- 

 ticles ; temperature, of course, being the 

 same in the two cases? Van't Hoff discov- 

 ered this remarkable fact ; that the gas- 

 pressure exerted by a gas particle is exactly 

 equal to the osmotic pressure exerted by a 

 dissolved particle, concentration and tem- 

 perature being the same in the two cases ; 

 space playing a role with gases, which is 

 analogous to that of the solvent with solu- 

 tions. In a word, Avogadro's law for gases 

 applies directly to the osmotic pressure of 

 solutions. The three fundamental laws of 

 gas-pressure thus apply directly to the os- 

 motic pressure of solutions. 



This relation is, in itself, of course very 

 interesting. But why is it referred to as of 

 epoch-making importance ? Partly because 

 of the new light which it throws on the 

 whole problem of solution. We can apply 

 thermodjmamics freely to gases, and since 

 the laws of gases apply to solutions, we can 

 use thermodynamics in dealing with solu- 

 tions in the same sense as in dealing with 

 gases. And partly because it is the forerun- 

 ner of the most important theory which has 

 been proposed in chemistry or in physical 

 chemistry for the last half- century. 



We have stated that the laws of gas- 

 pressure apply to the osmotic pressure of 

 solutions, and this is true for solutions of 

 certain classes of substances. It holds for 

 all of those substances which, when in solu- 

 tion, do not conduct the current and there- 

 fore do not undergo decomposition. These 



