Of all the fluid secretions, the secretion of Urine is perhaps the only important one which cannot cease to be fluid without immediate damage to the system. The deposit of solid matter from this secretion is dangerous, because tending to the formation of a solid calculus in the kidney or the bladder, in neither case easily extracted, and acting like a foreign body in these sensitive organs. This urine, naturally clear and limpid, contains in it several substances which are by their nature insoluble, but are held in solution by certain other materials. But in some morbid states these latter materials may be wanting, or else the insoluble bodies may be secreted in such quantity that the solvent material is unable to hold them in solution. In such instances, these parts of the urine may either be separated by the kidneys from the blood in a solid state, or may be deposited from the urine after excretion or on cooling. They then fall down in a crystalline or finely divided state, and constitute Urinary Deposits. Solvents are medicines which are employed to hold these insoluble substances in solution, where there is not enough of the natural solvent material in the system. They are medicines which tend, after being absorbed, to pass out of the blood into the urine. Although we are ill-informed as to the nature of the ordinary urinary solvents, yet it is evident that they must be present, because substances which are by their nature insoluble occur in healthy urine. And it is also evident that these medicines are able to supply their place; for, after one is given in a case of urinary deposit, this latter disappears, at the same time that the solvent remedy may be detected chemically in the urine. In this, the last order of Restorative Hæmatics, a number of apparently dissimilar medicines are grouped together, all of which agree in this point of their action. The deposits in which solvents are appropriate are termed respectively Lithic and Phosphatic. Among the first are comprehended Uric acid (also called Lithic,) Urate of Ammonia, and the more rarely occurring Urate of Soda. In the second set are comprised the Triple Phosphate of Ammonia and Magnesia, and the Phosphate of Lime. These deposits are each known by their peculiar form under the microscope.[35] They may easily be separated from the urine, when in any amount, and tested chemically. The Lithic deposits, (_i.e._ Uric acid and Urate of Ammonia,) are entirely dispelled by a red heat, and are soluble in alkalies. The Phosphatic deposits leave an earthy residue when heated, and are soluble in acids. When thus held in solution, the former are precipitated by an acid, the latter by an alkali, because by such a reagent that solvent is neutralized in each case.