Acids are used to correct a phosphatic deposit in the urine, caused by an alkalinity of that secretion. The alkaline urine may be secreted so, as has been observed in petechial fever by Dr. Graves and Dr. Golding Bird, and in insanity by Dr. Sutherland, and may also occur in diseases of the nervous centres; or it may be caused by a decomposition taking place in the bladder, as in chronic inflammation, or in the case of retention of urine from any cause. In the former case the acid may act as a corrective to the fluids before secretion; in the latter case, after it. But it is not always easy to cause acidity of the urine by any medicines. Mineral acids may be excreted in other ways, and vegetable acids are liable to decomposition in the system. (_Vide_ page 125.) The use of mineral acids in assisting a weak digestion admits of a simple explanation. For whatever notion we adopt as to the composition of the gastric juice, it is certain that it contains an acid in excess. Now an acid medicine would set free in the blood more of this acid which it is the business of the stomach to furnish, and thus prove useful in that kind of dyspepsia which depends on a failure of the gastric secretion. Hydrochloric acid has been particularly recommended by those who consider it to be the acid normally secreted by the stomach. When not wanted in the system, it seems probable that acids pass in all cases out of the blood in the same condition as they entered it. Thus vegetable acids act as diuretics; and mineral, as astringents to the glands generally. The latter, when given in excess, may prove hurtful by causing a lithic deposit in the urine. The addition of a mineral acid to healthy urine causes after some time a deposit of uric acid. The action of acids on the urine is neither so constant nor so certain as that of alkalies. In attempting to correct abnormal conditions of that secretion, it must be remembered that its reaction is liable to great variations in health. The whole amount passed during a day should be examined together. Dr. Bence Jones states that the urine is most alkaline just after meals, and most acid when a sufficient time has elapsed for the completion of the digestive process. (_Animal Chemistry_, p. 51.) I shall have afterwards to consider the action of the mineral acids as Astringents; and I must now add a few words on that use of the vegetable acids which has gained for them the title of Refrigerants.