Notices respecting New Boohs. 145 



together, and thus the force exerted on any particle of the gas is 

 the sum of the forces due to the motion of the surrounding gas 

 and the cohesion-pressures produced by the molecules in its neigh- 

 bourhood, these latter depending on the nature of the molecules as 

 well as on their distance apart. The result is that different gases 

 show unequal deviations from the laws of a perfect gas ; and by 

 compressing them sufficiently the cohesion-pressure is increased 

 until they finally become liquids. Van der Waals, proceeding on an 

 assumption, which he justifies, that these cohesion-pressures, 

 whatever be their absolute magnitude, must be proportional to the 

 square of the density of the gas, has elaborated a theory of gases 

 which accounts for the phenomena observed during their liquefac- 

 tion, and offers the most complete explanation of their behaviour 

 which has yet been given. 



In order to obtain a more exact knowledge of these cohesion- 

 pressures, observers began to study the properties of liquids, and 

 more especially of solutions of solids in liquids. During the past 

 ten years much work has been done, and a completely new theory 

 of solutions has been started by van 't Hoff, Arrhenius, Ostwald, 

 and other workers on the Continent. The object of the present 

 treatise is to give a concise account of this theory, of the evidence 

 on which it is based, and the results to which it leads. 



The older theories of solution are all based on the assumption 

 that the solvent and dissolved substance form definite chemical 

 compounds or hydrates, and therefore that the molecules of the 

 solution have a very complicated structure. The structure of these 

 molecules will change abruptly from one hydrate to another as the 

 amount of dissolved substance is increased; and so the properties 

 of the solutions will undergo similar sudden changes. The pre- 

 sence of such discontinuities in curves of density, freezing-point, 

 and other properties as related to concentration, has been vigorously 

 asserted by Mr. 8. U. Pickering in the pages of this Magazine, 

 w-hereas the upholders of the new theory deny the existence of any 

 discontinuity. 



According to the new theory, the molecules of the dissolved 

 substance behave quite independently of the solvent, the latter 

 playing an extremely subsidiary part. In addition to this, if the 

 solution be a dilute one the molecules of dissolved substance act 

 independently of one another, because of their relatively great dis- 

 tance apart. They are just like the molecules of a perfect gas. 

 If we take a partition which is permeable by the solvent and not 

 by the dissolved substance, the difference of pressure on its two 

 faces, when one side is in contact with pure solvent and the other 

 with solution, is due to the bombardment of the partition by the 

 molecules of the dissolved substance. Hence osmotic pressure, as 

 it is called, is accounted for. Similarly the extra pressure duo to 

 these moving molecules will cause the freezing-point and vapour- 

 pressure of the solution to fall below those of the solvent. In all 

 these cases the properties of the solution will depend on the 

 number of dissolved molecules and not on their const itution : so t hat 



Phil. Mag. S. 5. Vol. 83. No. 200. Jan, 1892. L 



