1 82 SCIENCE PROGRESS. 



On the assumption that dissolved substances exist as 

 simple molecules, and that the energy of translation of the 

 molecules is the same as in the case of a sras at the same 

 temperature, Boltzmann, Lorentz, and Riecke (1890-91), 

 each starting from a different point of view, have found, 

 by means of purely mechanical principles, that the pressure 

 of dissolved molecules should be equal to the corre- 

 sponding gaseous pressure. From thermodynamical con- 

 siderations, Planck and Duhem arrive at results which, in 

 the main, support the same conclusion, and van der Waals" 

 complex equation for a mixture of two substances, in the 

 case of a dilute solution, simplifies down to the gas- 

 equation. 



Dissolution and evaporation. — As consequences of the 

 new theory of solutions may be classed the views pro- 

 pounded by Nernst regarding the nature of the process of 

 dissolution, and the results which have been derived there- 

 from. The analogy between dissolution and evaporation 

 was pointed out by Dossios as long ago as 1867, and from 

 time to time it has been employed by writers on solution. 

 The conception of osmotic pressure for the first time 

 rendered this analogy definite. According to Nernst (18S9), 

 we may imagine that when a substance is in contact with a 

 solvent an expansive force is at work whereby molecules 

 pass into the solvent until the pressure they exert — the 

 osmotic pressure — reaches a definite value, which we may 

 term the solution-pressure. The process is thus taken to 

 be strictly comparable with that whereby a solid or a liquid 

 evaporates into a gas until its partial pressure reaches a 

 definite limiting value, the vapour-pressure. 



Diminution of the solubility. — Of the various conse- 

 quences which flow from this conception of the nature of 

 dissolution, the effect of a dissolved substance on the solu- 

 bility of a liquid may be discussed here. If the analogy 

 between dissolution and evaporation be valid, since the 

 vapour-pressure of a solvent is diminished by the presence 

 of a non-volatile dissolved substance, the solution-pressure 

 and hence the solubility of a liquid A in a solvent B should 

 be diminished by the presence in A of a substance insoluble 



