gravity increases, although on further diluting with water it again falls. Moreover (Mendeléeff, _The Investigation of Aqueous Solutions from their Specific Gravities_, 1887), the increase in specific gravity (_ds_), varies in all well-known solutions with the proportion of the substance dissolved (_dp_), and this dependence can be expressed by a formula (_ds_/_dp_ = A + B_p_) between the limits of definite compounds whose existence in solutions must be admitted, and this is in complete accordance with the dissociation hypothesis. Thus, 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 _ds_/_dp_ = 0·0729-0·000749_p_ (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, the definite compound C_{2}H_{6}O + 3H_{2}O, and others must be acknowledged in its solutions. The two aspects of solution above mentioned, and the hypotheses which have as yet been applied to the examination of solutions, although they have somewhat different starting points, 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 motions of molecules, which determine physical properties, depend on the motions and properties of atoms, which determine chemical reactions. For details of the questions dealing with 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 to dilute solutions only, whilst the chemical side deals mainly with strong solutions. In the consideration of the process of solution, besides the conception of diffusion, another fundamental conception is necessary--namely, that of the _saturation of solutions_.