Gravel may exist for years without inducing much disturbance of health, or it may produce serious inconvenience in a very short time. The urine being acid, holds the salts which it contains in solution; and, therefore, if the acid predominate, it becomes, through its own excess, thrown down, when it is detected in the form of red sand; but if there be an insufficiency of acid, the earths and salts of the urine are thrown down, and they exhibit their existence in the form of white gravel. Hence the two names “Red” and “White Gravel.” Acidulated urine is looked upon as certainly indicative of health, and when in excess, of a high tone of health; which, as the degree ascends, is marked by inflammation and fever. Alkaline urine betokens feebleness of constitution, or interrupted health. High living and an excited life induce red gravel; irregular, or an impoverished living, with much mental inquietude and physical exhaustion, establish white gravel. The fact of gravel being known to exist without forming stone, is no justification to suffer it to incur even the chance of such a finale. There is no class of diseases so painful and distressing as those wherein the function of the kidneys or bladder is interrupted, and the majority of these affections are ushered in by a derangement of the urinary fluid. _Cause of Gravel._—The cause of gravel is owing to chymical affinities. The urine, among its other constituents, contains _lithic acid_ and _ammonia_, which two, in healthy urine, are combined and held in solution. If any other acid, as may be instanced in expelled urine, possessing a greater affinity for the _ammonia_ than the _lithic acid_, be added, the _lithic_ will be thrown down. The same process takes place when effected in the system, the source of which new acid is the stomach, which, when in an irritable and feeble state, as in indigestion, furnishes or abounds with muriatic acid. In like manner, the urine containing salts, called phosphate of lime, ammonia, and magnesia, on receiving an additional quantity of ammonia,[18] the lime, for the less affinity to phosphoric acid than the ammonia, is thrown down; and hence a salt, bearing its name, is generated, either to be expelled like gravel, or to feed a nucleus already existing in the bladder. These chymical changes are produced by the causes before enumerated. We are subject to an infinitude of laws: we are perpetually changing, and these changes may fairly be stated as chymical affinities: it is owing to such, that the absorption of fluids, and the deposition of substances, which exist more or less in all derangements of health, ensue—to such, that we resist death, and to such that we become its prey. Gravel is a disease not so productive of fear or suffering, so long as it escapes when formed; but there are so many circumstances that may give rise to the formation of stone, that sandy urine should always command notice and treatment. A clot or