Mercury, Antimony, and Iodine, which increase more or less all the animal secretions, act, among others, upon the secretion of urine. Iodine particularly is said to be diuretic. Mercury and Antimony are most efficacious as indirect agents; the former by stimulating the function of the liver when impaired, the other by diminishing the pressure on the vessels in febrile cases. A large number of Diuretics are themselves secreted by the kidneys, and have been proved to pass out into the urine. A great many have been detected in the urine by different chemists, especially by M. Wöhler. The Carbonate, Nitrate, and Chlorate of Potash, and the Iodide of Potassium, have been found there. So also have the mineral acids--the vegetable acids--Magnesia--Mercury,[46] in combination--and Iodine, in the form of Hydriotic acid. Among vegetables, the principles of Chimaphila and Uvæ Ursi--the oils of Turpentine and Juniper, somewhat altered in nature--the oil and resin of Copaiba--and the acrid principle of Cubebs, all pass into it. Other substances, which are not diuretic, but astringent, have been found in the urine; as Alum, Lead, and the Morphia and Meconic acid of Opium. When a powerful Diuretic is given in too large a dose, it may cause a degree of action sufficient to produce congestion of the kidney, and so defeat the object for which it was intended. For congestion of a gland is invariably followed by a diminution or stoppage of its secretion. So a large dose of Turpentine or Cantharides may cause a dangerous attack of strangury, or even a total suppression of the urine for a time. (_Vide_ p. 274.) It can hardly be said that Diuretics are of very extensive application in the treatment of disease, or that they are medicines which can ever be much relied upon. For not only are the remedies themselves uncertain in action, but it happens also that the disorders in which they are most required are of a peculiarly obstinate nature. There are two chief actions for which Diuretics may be required. They may be used: (1.) to eliminate solid materials from the blood; (2.) to promote absorption, by diminishing the amount of fluid in the blood. It is easier to employ the first than to exert the second of these operations.