_As a diluent_, water is indispensable to the preservation of health. The body being composed of solids and fluids, there must be maintained a certain relative proportion of these, to constitute that state of system called health. In a full grown adult, the solid matter of the body, by which we mean all that substantial part of the frame which is not in constant motion in the vessels, amounts to only about one fifth of the weight of the body--Chaussier says, one ninth of the total weight, the difference, perhaps, being owing to the fact that there is a quantity of fluid combined with the solids in so intimate a manner, as almost to constitute a part of their substance. The diminution of the fluid part of the body, is the cause of an uneasy sensation, indicating the necessity of repairing the waste of fluids, which we familiarly call _thirst_. This is a sensation connected with some natural state of the corporeal functions, and altogether independent of the occasional excitement of foreign bodies, although it may be induced by these. There is a demand for a certain supply of liquid which is the result of repletion of the stomach, and the cause of our drinking at our ordinary meals, but this is different from true or spontaneous thirst. True thirst occurs, when we have been some time without taking drink, (unless the food has consisted mainly of fruits and other succulent vegetables; under which circumstances, a person may go for months without any desire for drink); when the system has been greatly excited, whether by corporeal or mental causes; when acid substances, particularly saline bodies, have been taken into the stomach; and, in short, in every condition of the system, which favors the inordinate excretion of fluids. The immediate cause of thirst appears to be a dry state of the mouth and fauces; owing to the mucus which covers these parts becoming thick and viscid, though physiologists are not agreed on this point. This may arise from the absorption of the fluid parts of the saliva; for it appears to be necessary for the due performance of the functions of the palate and the tongue, that the mucus should possess a certain degree of liquidity. The sensation of thirst is generally indicative of the necessity of a supply of fluid to the system generally; for although thirst may be momentarily assuaged by wetting the mouth, or holding a thin fluid in it--yet it can only be effectually relieved by conveying into the stomach a quantity of fluid sufficient to supply the deficiency. This supply is termed _dilution_, from the fact that the fluid is absorbed and carried into the blood, which it renders thin, and the fluids themselves are called _diluents_.