but by increasing its proportion of _Tartaric Acid_, we convert it into a _super-tartrate_ or “_Cream of Tartar_,” which is a substance characterized by a comparative degree of insolubility, and a correspondent change is produced in the medicinal activity of the salt; its purgative effects are considerably diminished, whilst its diuretic powers are rendered more considerable. We may even extend this experiment by adding to the _Cream of Tartar_, _Boracic Acid_, a substance capable of increasing to a certain extent its solubility, when we shall again find that its purgative properties are strengthened in an equal proportion. It has been observed that a mixture of different saline cathartics is more efficient than an equivalent dose of any single one, a fact which is strikingly exemplified in the prompt and active operation of Cheltenham Salts, in comparatively small doses, as well as in that of sea water. I submit whether this may not in some degree depend upon increased solubility; for it is a law well known to the chemist, that _when water has ceased to act upon a salt_, _in consequence of its having obtained the term of saturation, the solution may still take up another salt of a different kind_. I apprehend that an advantageous application of this law might be frequently made in practice, and the energies of a remedy thereby considerably extended.[267] Where the active principle of a cathartic is not sufficiently soluble, it is apt to vex and irritate the bowels, producing tormina instead of exciting a free and copious excretion; hence the reason why the operation of resinous purgatives is so commonly attended with griping, and why relief may be obtained by combining them with _neutral salts_. Thus also _Senna_, whose virtues reside in extractive matter, is apt by decoction, or long exposure to the air, to act with griping, in consequence of the extractive matter becoming by oxidation, resinous and comparatively insoluble: this effect is best counteracted by the addition of _soluble Tartar_, that will quicken its action, or by an alkaline salt that will increase its solubility. It appears then to be established as a pharmaceutical maxim, that _the intensity and even specific action of a purgative medicine may be modified or completely changed_, _by changing the degree of solubility possessed by the principles in which its activity resides_. The application of this principle is highly important in practice, directing us in the choice of the different purgatives, according to the objects which we may wish to fulfil by them, and pointing out safe and easy methods by which we may increase, diminish, retard, or accelerate their operation; it thus enables us to construct new and powerful combinations, by imparting to established remedies fresh activity, or by mitigating the acrimony and violence of arrangements in other respects efficacious and eligible.