Water is the simplest and most efficacious of all solvents, and should in all cases of gravel be very freely administered. Bouchardat has remarked that great water-drinkers are never afflicted with stone. All the urinary deposits are, to some extent, soluble in water, although _comparatively_ insoluble. Whenever the urine is of higher specific gravity than it should be in health, more water should be drunk. The average specific gravity of healthy urine is about 1.018.[38] Acids and Alkalies are most important as solvents. They do not now act indirectly, as in the case before mentioned, but directly dissolve the insoluble matter. Acids dissolve a phosphatic, and Alkalies or their carbonates a lithic deposit, in the body, as well as out of it. Although the natural solvent may be neither an acid nor an alkali, but something else different in nature from these, yet any thing that will rightly dissolve the deposit will be fit to supply its place. Any of the free Acids may serve to dissolve a precipitated phosphate. Hydrochloric is perhaps the best of the mineral acids, although Phosphoric has been recommended on theoretical grounds. Sulphuric is the least efficacious because it does not always pass out in the urine. (_Vide Chap. IV._) Sour fruits, as Currants or green Gooseberries, may be useful in phosphatic cases; but ripe fruits have an opposite tendency. Acidulous drinks may be recommended, as Cider, Perry, and Rhenish wines. Of the free alkalies used to dissolve a lithic sediment, Potash is preferable to Soda, because it forms a more soluble compound with Uric acid. The Bicarbonates of the fixed alkalies are best, because least irritant. Bicarbonate of Soda is contained in Vichy water. The soluble Bicarbonate of Magnesia may be given with advantage. The Carbonate of Lithia has been recommended by Mr. A. Ure; but it is comparatively insoluble. The neutral vegetable salts of the alkalies, and sweet fruits which contain them, are equivalent to the Carbonates, because converted into them in the system, as has already been shown. They may be pleasantly administered in the form of effervescing draughts, in the formation of which the Carbonic acid of an alkaline carbonate is displaced by Citric or Tartaric acid. The danger which attends the continual use of acids, or of alkalies, particularly of the latter, is that their long employment may engender a condition of an opposite nature to that which they were intended to alleviate. An acid may at length cause a lithic deposit in the urine; or, still more frequently, an alkali may produce a phosphatic sediment. Their administration should therefore be conducted with caution.