Now the circumstances which may cause these deposits are mainly of four kinds. (1.) A wrong in the diet. (2.) An error in the normal reaction of the blood, causing these matters to be deposited, _without being themselves in excess_. (3.) The suppression of another secretion. (4.) A fault of some process of assimilation or secretion, causing _an absolute excess_ of these constituents of the urine. Urinary sediments may be caused by slight variations in diet. Excessive indulgence in animal food or in wine may cause an over-secretion of Lithic acid. Sour drinks may cause a similar deposit, by rendering the urine acid; and sweet fruits, containing vegetable salts of the alkalies, may produce a phosphatic sediment, by rendering it alkaline. Such cases may be remedied by an attention to diet. Again, deposits may be caused by an excess of acid or of alkali in the blood, which excess is excreted by the kidneys, and causes a lithic or phosphatic gravel, without an excess of Lithates or of Phosphates in the urine. Whatever be the condition of Uric acid in urine, it is certain that it is held in solution by some matter of an alkaline nature.[36] When this alkaline matter is neutralized by an acid, the Uric acid precipitates. This may perhaps be caused in Rheumatic fever by an excess of Lactic acid. The Phosphates are held in solution by Phosphoric acid, or by that matter which gives to healthy urine its slight acid reaction.[37] An excess of alkali in the blood, which may probably occur in Typhoid and low fevers, will cause their precipitation. Or the alkali may be formed in the bladder by the decomposition of the urine. This may occur in inflammation of the bladder; or in retention of urine caused by paraplegic paralysis. In such cases there is no absolute excess of the deposited matter; but it is in relative excess, for the substance which should properly dissolve it is neutralized by a reagent of an opposite nature. Acids and alkalies have already been mentioned as efficacious in these instances; the former in phosphatic, the latter in lithic deposits. They directly neutralize the disturbing cause; the alkali, or the acid. They are not in such cases truly solvents; they do not themselves hold in solution the insoluble material, but they set free something else that shall be capable of doing so. (_Vide Acids; Alkalies._) A suppression of the secretion of the skin causes a lithic deposit in some cases, as in a common "cold." For the perspiration contains a free acid, probably lactic or butyric; and when it is suppressed, the secretion of this acid is thrown upon the kidneys, and the urine is rendered more acid than naturally. The obvious indication in the treatment is to restore the function of the skin, as by warm baths, diaphoretics, etc. It is in the case of urinary deposits produced in the fourth way that true solvents are appropriate.