_True Diuretics._--Though all true Diuretics pass into the urine, the converse of this, _i.e._ that all matters which pass into the urine are Diuretic, is not always true. Thus the astringent mineral salts pass frequently out of the system in the urine, but seldom affect its amount in either way. And for the reasons already enumerated, it is often impossible, even by the most powerful medicines of this order, to increase the urine beyond a certain amount. Diuretics may be somewhat loosely arranged in four groups:--1. Water, and soluble mineral substances, _i.e._ acids, alkalies, and salines under certain conditions; 2. Acrid matters of various kinds; 3. Alcoholic and ethereal liquids; 4. The minerals which increase all secretions. Diluents promote the secretions of the skin and kidneys. A certain quantity of water must be given with every diuretic dose, and the larger the quantity the greater will be the effect produced. In the case of a Diaphoretic, this is all that is required. But as Diuretics are generally given, as in dropsies, for the purpose of eliminating fluid out of the system, this object would be clearly defeated by the introduction of a large quantity of water into the system. This, therefore, should be avoided. The free acids, both mineral and vegetable, with the exception of Sulphuric acid (_vide_ p. 262,) pass into the urine, and act as Diuretics. So also do the mineral alkalies. But as these medicines have important agencies in the blood, and tend further, by altering the reaction of the urine, to produce in it deposits of different kinds (_vide_ p. 151,) they can seldom be safely employed for this purpose. Salines are more appropriate. They should not be given in large doses, for they will then act upon the bowels. The dose should be small, and moderately diluted with water. To prevent it from acting upon the skin, the conditions which favour diaphoresis should be, if possible, avoided. The subject of the action of saline medicines, and the effect of dilution upon this action, has been already considered. (p. 74.) The acrid Diuretics are perhaps the most powerful medicines in the order. One of them, Cantharides, is an animal product. Some contain or consist of volatile oils: as Juniper, Turpentine, Cajeput, Copaiba, Horseradish. These volatile oils may act upon the skin instead of the kidneys, under the conditions which are mentioned above as favouring diaphoresis. Others of this group contain peculiar vegetable principles; as Broom, Chimaphila, Taraxacum, Colchicum, and Squill. Alcohol, Ether, and Nitric ether, act powerfully both on the kidneys and on the skin. Wine, which further contains certain volatile oils, and sometimes a vegetable acid, acts on the kidneys when drunk in any quantity. The diuretic effect of these liquids is, as in the case of salines, greater when the surface is cold, and less when it is warm and covered.