[69] 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 _An_H_{2}O, 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 _An_H_{2}O, 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 _An_H_{2}O, or they may introduce conditions for the possibility of building up new systems _Am_H_{2}O, 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 _Am_H_{2}O, then the addition of fresh molecules of water or of _A_ would be followed by the formation of new molecules _An_H_{2}O. 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, _An__{1}H_{2}O and _Am__{1}H_{2}O, 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 Na_{2}SO_{4} + 10H_{2}O, or Na_{2}SO_{4} + 7H_{2}O, or Na_{2}SO_{4}, are contained in solutions. The comparatively more stable systems _An__{1}H_{2}O 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 _n__{1}H_{2}O; the property also and state of systems _An_H_{2}O and _Am_H_{2}O, occurring in solutions, is that they are in a liquid form, although partially dissociated. Substances _A__{1}, which give solutions, are distinguished by the fact that they can form such unstable systems _An_H_{2}O, but besides them they can give other much more stable systems