Conditions of Molecular Action. 365 



Professor Wanklyn* has pointed out the influence of adhesion 

 and of vapour-density upon the distillation of mixtures, by which 

 means a less volatile liquid can be made to distil faster than one 

 more volatile with which it is mixed — as, for instance, in the 

 facilitation of the distillation of oils in steam ; and Dr. Graham f 

 seems to think that nitrogen may diffuse more readily through 

 a platinum plate in company with hydrogen than without its 

 assistance. 



It seems probable that the rate of diffusion of gases through 

 various porous substances is affected by the molecular affinities 

 of the solids and gases, as well as by the constitution of the gases 

 themselves J. 



Solution. — The solution of solids in different solvents, without 

 change of chemical composition, must be similar in its nature to 

 mixture ; but in this case a species of liquefaction (or rather dis- 

 integration) must precede mixation. In many instances the action 

 is accompanied by an absorption of heat from surrounding objects, 

 showing that a change of state similar to liquefaction has taken 

 place. 



If, however, solution involved true liquefaction, the solvent 

 ought to lose exactly that equivalent of heat which is required to 

 melt or liquefy the substance dissolved, which is not the case. 

 But if the particles of the solvent simply come into such close 

 contact with the particles of the solid as to disintegrate and 

 overcome their cohesion to their fellows, then it is by no means 

 necessary that any heat should disappear from the solution. 



The action of a most minute portion of alcohol in assisting the 

 pulverization of camphor testifies the probability of this expla- 

 nation. 



Heat increases the distance from one another of the particles 

 of a solution ; it may also in some instances directly interfere 

 with molecular attraction, and thus a larger quantity of a solid 

 can be dissolved without bringing its separated particles within 

 the range of their own cohesive or crystallizing forces §. 



When a substance, such as hydrate of lime, is more readily 

 dissolved in cold than in hot water, it seems probable that its 

 molecular affinity for the solvent is so small that it requires the 

 combined influence of many particles of the solvent condensed 

 upon its molecules to keep them in suspension. When these sol- 

 vent particles are separated from one another by heat, the mole- 



* Phil. Mag. S. 4. No. 183. 



t " On the Absorption and Dialytic Separation of Gases by Colloid 

 Septa," Phil. Trans, 1866, p. 420. 



X Dr. Graham, ibid. p. 431. 



§ Mr. Tate has shown that the weight of a drop of fluid is diminished 

 by heat. 



