<>N YYATKK AND ITS COMPOUNDS 65 



In the consideration of the process of solution, besides the con- 

 ception of diffusion, another fundamental conception is necessary, 

 namely, that of the saturation of solutions. 



^pecial and particular attraction or affinity between them. A vapour or gas permeates 

 into any other vapour or gas, but a salt which dissolves in water may not be in the least 

 soluble in alcohol, and is quite insoluble in mercury. In considering solution as a mani- 

 festation of chemical forces (and of chemical energy), it must be acknowledged that they 

 an- here developed to so feeble an extent that the definite compounds (that is, those 

 Formed according to the law of multiple proportions) which are formed between water 

 and a soluble substance dissociate at even the ordinary temperature, forming a homo- 

 geneous system that is, one where both the compound and the products into which it 

 decomposes (water and the aqueous compound) occur in a liquid state. The chief diffi- 

 culty in the comprehension of solutions depends on the fact that the mechanical theory 

 of the structure of liquids has not yet been so fully developed as the theory of gases, and 

 solutions are liquids. The conception of solutions as liquid dissociated definite chemical 

 compounds is based on the following considerations : (1) that there exist certain undoubt- 

 edly definite chemical crystalline compounds (such as H 2 SO 4 , H 2 O ; or NaCl, 10H 2 O ; or 

 CaClo, 6HoO ; ivrc.) which melt on a certain rise of temperature, and then form real solu- 

 tions ; (2) that metallic alloys in a molten condition are real solutions, but on cooling they 

 often give entirely distinct and definite crystalline compounds, which are recognised by 

 the properties of alloys; (3) that between the solvent and the substance dissolved there 

 are formed, in a number of cases, many undoubtedly definite compounds, such as com- 

 pounds with water of crystallisation ; (4) that the physical properties of solutions, and 

 especially their specific gravities (a property which is very accurately observable), vary 

 with a change in composition, and in such a manner as the formation of one or several 

 definite but dissociating compounds would require. Thus, for example, on adding 

 water to fuming sulphuric acid its density is observed to decrease until it attains the 

 definite composition H 2 SO 4 , or SO 3 + H 2 O, when the specific gravity increases, although 

 on further diluting with water it again falls. Further (Mendeleeff, The Investigation of 

 Aqueous Solution* from their Specific Gravities, 1887), the increase in specific gravity 

 (ds), with the augmentation (dp) of the percentage amount of a substance dissolved, 

 varies in all well-known solutions with the percentage amount of the substance dissolved, 



so that a rectilinear dependence is obtained (i.e., ( S = A + B) between the limits of 



dp 



definite compounds which must be acknowledged to exist in solutions ; this would be 

 expected to be the case from the dissociation hypotheois. So, for instance, from H 2 SO 4 

 to H 2 SO 4 + H 2 O (both these substances exist as definite compounds in a free state), the 



fraction ( S = 0'0729-0'000749p (where p is the percentage amount of H 2 SO 4 ). For 



alcohol C 2 H 6 O, whose aqueous solutions have been more accurately investigated than all 

 others, three definite compounds must be acknowledged in its solutions, C 2 H C O -f 12H 2 O, 

 C 2 H ( ,O + 3H 2 O, and 3C 2 H 6 O + H 2 O. 



The two aspects of solution above mentioned, and the hypotheses which have as yet 

 been applied to the examination of solutions, although they have partially different 

 starting points, yet will doubtless in time lead to a general theory of solutions, because 

 the same common laws govern both physical and chemical phenomena, inasmuch as the 

 properties and movements of molecules, which determine physical properties, are depend- 

 ent on the movements and properties of atoms, which determine chemical mutual actions. 

 For details of the questions dealing with the theories of solution recourse must now be 

 had to special memoirs and to works on theoretical (physical) chemistry; for this subject 

 forms one of special interest at the present epoch of the development of our science. 

 In working out chiefly the chemical side of solutions I consider it to be necessary to 

 reconcile the two aspects of the question; this seems to me. to be all the more possible, 

 as the physical side is limited t,o dilute solutions only, whilst the chemical side deals 

 mainly with strong solutions. 



VOL. -I. P 



