November 10, 1911 ] 



SCIENCE 



623 



and retains its properties unchanged. An 

 example of this type of solution may be 

 found in the solution of one saturated 

 hydrocarbon in another, say of pentane in 

 hexane. On mixing the two liquids there 

 is no evidence of union between them, the 

 volume of the mixture is practically the 

 sum of the volume of the components, the 

 heat of solution is practically nil, the va- 

 por pressure of each constituent is reduced 

 merely as if by dilution with the other con- 

 stituent, and so on. That there is some 

 action between the two components even in 

 this extreme case must be admitted, but it 

 may be referred entirely to action of a 

 physical kind, such as one finds on mixing 

 one gas with another at considerable pres- 

 sures. Action of a chemical nature is ab- 

 sent. If it be said that even saturated 

 hydrocarbons have some chemical affinity 

 for each other, recourse may still be had for 

 examples to mixtures of two inactive ele- 

 ments, say liquid argon and liquid kryp- 

 ton, where chemical affinity is non-existent. 



At the other extreme we have such solu- 

 tions as those of sulphuric acid and water. 

 Here there is every physical evidence of 

 chemical union. The volume of the mix- 

 ture is by no means the sum of the volumes 

 of the components, the amount of heat 

 evolved on mixing is very great, the sepa- 

 rate liquids, which are practically non-con- 

 ductors, yield on mixing a solution which is 

 a good conductor, and so on. There is ob- 

 viously here a great infiuenee of the solvent 

 water on the solute sulphuric acid, and this 

 influence we can only account for by as- 

 suming that it is essentially chemical in 

 character. 



As the influence in such a case is neces- 

 sarily reciprocal, then if even one of the 

 constituents of the solution is inactive 

 chemically there can plainly be no action 

 of a chemical nature on mixing. Thus, no 

 matter what solvent we take, it can exercise 



no action other than that of a physical kind 

 on argon, say, which has been dissolved in 

 it; and, again, if liquid argon is chosen as 

 solvent no substance dissolved in it can be 

 affected by it chemically, and we thus ob- 

 tain only the properties of a physical mix- 

 ture. It is convenient therefore to classify 

 liquid solvents according to their chemical 

 activity. The saturated hydrocarbons, 

 which are chemically very inert, and, as 

 their name paraffin implies, little disposed 

 to chemical action of any kind, may be 

 taken as typically inactive solvents, anal- 

 ogous to liquid argon. Water, on the other 

 hand, as its numerous compounds (hy- 

 drates) with aU kinds of substances testify, 

 may be taken as a typically active solvent. 

 The ordinary organic solvents exhibit inter- 

 mediate degrees of activity. 



For the purpose of illustrating the effect 

 of solvents on a dissolved substance one 

 may conveniently take a colored substance 

 in a series of colorless solvents. If the sub- 

 stance is unaffected by the solvent, we 

 might reasonably expect the color of the 

 solution to be the same as the color of the 

 vapor of the substance at equal concentra- 

 tion. Iodine, for instance, gives rise to the 

 familiar violet vapor. Its solution in car- 

 bon disulphide has a color practically simi- 

 lar, but its solution in alcohol or water is of 

 a brown tint quite different from the other. 

 In the indifferent hydrocarbons and in chlo- 

 roform the color is like that in carbon disul- 

 phide, in methyl or ethyl alcohol it is 

 brown. We conclude therefore roughly 

 that iodine dissolved in saturated hydro- 

 carbons, in chloroform, carbon tetrachlo- 

 ride and carbon disulphide is little affected 

 by the solvent, whereas in water and the 

 alcohols it is greatly affected, probably by 

 way of combination, since in aU the solvents 

 two atoms of iodine seem to be associated 

 in the molecule. That combination between 

 the iodine and the active solvents has really 



