"Crystallized sediments, or red gravel, consist of lithic acid, nearly pure. Lithic acid, as has been before stated, exists in a state of combination in healthy urine; and in such a proportion, as to be held in a state of solution at all ordinary temperatures. Sometimes, however, a free acid is generated by the kidneys, which precipitates the lithic acid in the pure crystallized state we see it--a phenomenon easily imitated artificially, as is well known, by the addition of a few drops of any acid to healthy urine. The precipitation of crystallized lithic acid does not, therefore, necessarily indicate an excess of lithic acid in the urine, but the presence only of some free acid in that fluid; though such an excess does, for the most part, exist in this form of disease, as will be shown hereafter. With respect to the nature of the precipitating acid, it is probably not always the same. Most generally it appears to be the _muriatic_, sometimes the _phosphoric_ or _sulphuric_, and occasionally other acids. In general, however, it is to be understood, as noticed elsewhere, that when the mineral acids are present in excess, these are the _immediate_ cause of the preternatural acidity in the urine, and consequently of the precipitation of the lithic acid. The stronger acids act by decomposing saline compounds, into which destructible acids, such as the lactic acid, &c. enter, and setting them free. Hence the _immediate_ cause of the deposition of lithic acid gravel is generally a destructible acid of very weak powers: even, perhaps, in some instances, the carbonic acid. When the urine contains a free acid, it is commonly more transparent than usual, and of a bright copper colour." p. 112. The treatment recommended by Dr. PROUT in this species of gravel is as follows: First, a strict attention to diet, avoiding the hurtful articles already enumerated. Secondly, the use of _alkaline_ remedies; but those must not be depended upon, without the aid of other means, more especially of alteratives and purgatives. Accordingly we are informed that "The pil. submur. hydrarg. comp., or a pill composed of the pil. hydrarg. and pulv. antimonialis, may be taken twice or thrice a week at bed time, and followed up the next morning by an active dose of the sub-sulphate of magnesia, or a mixture of Rochelle salts and magnesia, or carbonate of soda. A little of either of these compounds may be also taken twice or thrice in the day, so as to keep the urine constantly neutral or alkaline, and the bowels freely open; or gr. x to xx of magnesia may be taken for the same purpose in a glass of soda water, as often as it may be found necessary."