The medicinal or mineral waters participate more or less of the earthy and saline substances found in common waters; with generally some prevailing ingredient from which they have received their names.

1. Aquae Alkalinae. Alkaline Waters, As That Of Tilbury

The waters of this kind are impregnated chiefly with the mineral alkaline salt, and with calcareous earth •, both which readily discover themselves in the residuum left upon evaporation.

The Tilbury water, one of the strongest perhaps of this class, has been found serviceable, not only in complaints arising from acidities in the first passages, but likewise in obstinate alvine fluxes, some cutaneous defedations, female weaknesses, and other disorders from a laxity and debility of the fibres: it generally passes off freely by urine or perspiration, and sometimes, on first taking, purges a little. It may be drank to the quantity of a quart a day, or more; cold, or just warmed; by itself, or with the addition, if requisite, of milk, with which it perfectly agrees. It does not bear evaporation, or a boiling heat; soon growing milky, and depositing a part of its earth and virtue. In close vessels, it keeps well: some that had been carried to the coast of Guinea and brought back again, appeared to me unaltered in all its properties.

2. Aquae Catharticae Amarae. Bitter Purging Waters

These waters are distinguished by their bitter taste; and by their depositing, on the addition of alkaline salt, a copious white earth, great part of which is found to be magnesia. The dry matter, left upon evaporating them, con-sists of the sal catharticus amarus, intermixed with different earths, and often a small proportion of other saline matters. The quantity of salt differs in different waters; some yielding scarcely two drams, and others an ounce or more on the gallon. One of the strongest of the purging waters in this kingdom is that of Jeiffop's Well near Kingston, of which a pint left on evaporation seventy-two grains; Kilburn and Cheltenham waters yielded sixty-four grains; Acton, not fifty; and that of Epsom, less than forty.

These waters are mild and gentle purgatives, operating with sufficient efficacy, yet in general with ease and safety; rarely occasioning any gripes, nausea, or lowness. They may likewife be so managed as to promote the animal secre-tions in general, and prove excellent aperients and attenuants in sundry chronical disorders.

The dose of these waters, as a purgative, is from one to three pints, according to their strength, to be drank by a little at a time. Their virtue may be increased, by dissolving in the water some of the purified salt, or other purgative saline substances, as the artificial salt of Glauber, soluble tartar, Rochelle salt, or manna: additions of this kind are more eligible than boiling down the water, as its strength is augmented in a more certain ratio, and its na-tural constitution preserved entire. To render the liquor more acceptable to the palate and stomach, some grateful distilled water, or aromatic tin dure, as the tictura cardamomi, may be added: it has been customary to infuse or boil in the water some aromatic seeds, as those of caraway: but very little of the virtue of the seeds is by this treatment communicated to the water. Their taste is excellently covered by honey. As alterants, the water may be used for common drink; diluted with simple water, milk, whey, wine, or other liquids, so as but just to keep the belly open.

These waters are found to purge more in their natural state, than after they have been boiled, and than the salt obtainable from an equal quantity of them. They contain, besides the purging salt, no small proportion of calcareous earth. Now, if a solution of calcareous earth, made either in pure water or in acids, be mixed with a solution of the purging salt, and the liquor evaporated; great part of the salt will be destroyed, its acid being transferred from its own earth into the calcareous earth, and forming, with this, a concrete neither purgative nor dissoluble, namely selenites. As such a concrete is found in the dry residua of the purging waters, we may presume, that it owes its origin, as in this experiment, to the destruction of a part of the purgative ingredient; and that the water holds naturally a greater quantity of salt, than can be extracted from it by art.