Note 69 — Chapter I · ON WATER AND ITS COMPOUNDS

According to the above supposition, the condition of solutions in the sense of the kinetic hypothesis of matter (that is, on the supposition of an internal motion of molecules and atoms) may be represented in the following form:—In a homogeneous liquid—for instance, water—the molecules occur in a certain state of, although mobile, still stable, equilibrium. When a substance A dissolves in water, its molecules form with several molecules of water, systems AnH2O, which are so unstable that when surrounded by molecules of water they decompose and re-form, so that A passes from one mass of molecules of water to another, and the molecules of water which were at this moment in harmonious motion with A in the form of the system AnH2O, in the next instant may have already succeeded in getting free. The addition of water or of molecules of A may either only alter the number of free molecules, which in their turn enter into systems AnH2O, or they may introduce conditions for the possibility of building up new systems AmH2O, where m is either greater or less than n. If in the solution the relation of the molecules be the same as in the system AmH2O, then the addition of fresh molecules of water or of A would be followed by the formation of new molecules AnH2O. The relative quantity, stability, and composition of these systems or definite compounds will vary in one or another solution. I adopted this view of solutions (1887, Pickering subsequently put forward a similar view) after a most intimate study of the variation of their specific gravities, to which my book, cited in note 19, is devoted. Definite compounds, An1H2O and Am1H2O, existing in a free—for instance, solid—form, may in certain cases be held in solutions in a dissociated state (although but partially); they are similar in their structure to those definite substances which are formed in solutions, but it is not necessary to assume that such systems as Na2SO4 + 10H2O, or Na2SO4 + 7H2O, or Na2SO4, are contained in solutions. The comparatively more stable systems An1H2O which exist in a free state and change their physical state must present, although within certain limits of temperature, an entirely harmonious kind of motion of A with n1H2O; the property also and state of systems AnH2O and AmH2O, occurring in solutions, is that they are in a liquid form, although partially dissociated. Substances A1, which give solutions, are distinguished by the fact that they can form such unstable systems AnH2O, but besides them they can give other much more stable systems An1H2O. Thus ethylene, C2H4, in dissolving in water, probably forms a system C2H4nH2O, which easily splits up into C2H4 and H2O, but it also gives the system of alcohol, C2H4,H2O or C2H6O, which is comparatively stable. Thus oxygen can dissolve in water, and it can combine with it, forming peroxide of hydrogen. Turpentine, C10H16, does not dissolve in water, but it combines with it as a comparatively stable hydrate. In other words, the chemical structure of hydrates, or of the definite compounds which are contained in solutions, is distinguished not only by its original peculiarities but also by a diversity of stability. A similar structure to hydrates must be acknowledged in crystallo-hydrates. On melting they give actual (real) solutions. As substances which give crystallo-hydrates, like salts, are capable of forming a number of diverse hydrates, and as the greater the number of molecules of water (n) they (AnH2O) contain, the lower is the temperature of their formation, and as the more easily they decompose the more water they hold, therefore, in the first place, the isolation of hydrates holding much water existing in aqueous solutions may be soonest looked for at low temperatures (although, perhaps, in certain cases they cannot exist in the solid state); and, secondly, the stability also of such higher hydrates will be at a minimum under the ordinary circumstances of the occurrence of liquid water. Hence a further more detailed investigation of cryohydrates may help to the elucidation of the nature of solutions. But it may be foreseen that certain cryohydrates will, like metallic alloys, present solidified mixtures of ice with the salts themselves and their more stable hydrates, and others will be definite compounds.