e. Urea can only be discovered satisfactorily by evaporating the suspected fluid to dryness and treating the residue with alcohol, again evaporating this alcoholic solution to dryness, redissolving in water,and adding nitric or oxalic acid with the precautions to be mentioned hereafter when treating specially of the deter mination of urea. When common salt is al .lowed to crystallize from a moderately dilute solution of urea, the form of the crystals is modified, and instead of obtaining cubes or octohedm the crystals developed assume a more or less penniform appearance, often shew ing the figure of a Maltese cross with serrated edges. These modifications have been proposed as a test of the presence of urea : they are not, however, certain indications, though useful as affording a presumption of its presence. When a fluid which contains urea is concen trated by evapomtion, and nitric acid is then added, by spontaneous evaporation on a glass plate we obtain lamellar crystals of nitrate of urea in irregular rhomboidal plates with the acute angles often truncated.
j: To detect the existence of uric acid and the urates, if albuminous principles are present, the liquid is evaporated to dryness, and the residne digested for some hours with solution of caustic potash, till every thing soluble has been taken up. It is diluted, filtered, and supersaturated vvith hydro chloric acid ; a flocculent precipitate forms, which is redissolved by excess of acid ; and the uric acid separates; this is collected on a filter. We are enabled by this pro cess, where considerable quantities of uric acid are present, as in the excrements of birds of prey, to obtain results of considera ble accuracy ; but where the proportion of the acid is very minute, it cannot be relied upon for quantitative experiments ; the residue in these cases must be subjected to microscopic examination. If no azotised matters are pre sent, the mere addition of free hydrochloric acid after a lapse of some hours usually causes a separation of rhombic crystals of the acid ; or by evaporating to dryness and treating th residue with water acidulated with hydro chloric acid, the uric acid reinains undissolved the residue is proved to contain this compound, either by the appearance of its crystals unde the microscope, or by evaporating a small por tion to dryness with nitric acid on a slip o glass, when a red stain is left ; a drop of am monia added produces a fine crimson ; if be evaporated to dryness, a drop of solution of caustic potash converts it into a beautiful purple, which is destroyed by the applic-ation of heat.
The methods for testing the other acids will be described when treating of their quantitative• determination.
Having now obtained an idea of the nature of the fluid we have to exarnine, we are ready to proceed with its Quantitative analysis.
A thin porcelain or platinum capsule carefully counterpoised, and filled with a give cc eight of the fluid, usually 500 or 1000 grains.
This is evaporated to dryness in a water-bath. It is carefully weighed and the total solid con tents determined ; the loss indicates volatile niatters consisting almost entirely of water. The residue is carefully incinerated with the usual precautions until all traces of carbon are removed : on weighing again we obtain the fixed saline constituents.
By this means we have determined Water and volatile matters.
Organic matters and ammoniacal salts. Fixed saline matters.
The quantity of each organic substance has now to be estimated. We have at present no absolutely exact method of determining all the ingredients from one portion only of a fluid ; the following offers the nearest approximation.
a. Fibrin separates hy spontaneous coagula tion; it is washed and determined in the manner to be described hereafter.
b. If casein and albumen be both present, a given portion of the clear liquid is acidulated with a few drops of acetic acid, and evaporated to dryness in vacuo over sulphuric acid. The residue is digested with successive portions of water at 120°, as long as any thing• dissolves; this residue is then dried upon a water-bath, pulverised, and treated with ether, by which all fatty matters are removed ; casein alone remains behind. It is completely dried, weigh= ed, incinerated, and the ashes deducted. This method of separating the casein is imperfect, as a portion of this substance generally redissolves on the addition of water. If uric acid be pre sent, it will be found with the casein, which must be dissolved in solution of potash, diluted, filtered, supersaturated with acetic acid, by which the azotised matter at first precipitated is redissolved. Uric acid alone remains; it is collected on a weighed filter, and the quantity determined. It should always be tested by the microscope or by nitric acid.
c. The aqueous solution filtered from the casein, or the liquid for examination, if this principle be absent, is evapnrated to dryness by a water-bath. If much fat is present, the residue is washed with ether to i.move the greater part of the oily matter, then dried tho roughly, and powdered; an operation which, after the preliminary treatment with ether, may be effected without much difficulty. The pul verized mass is digested with ether, and the ethereal solutions, including that in which the casein was digested, are mingled and evapo rated to dryness. Cholesterin, fatty matters, lactates, and a trace of inlea are obtained; by digesting with water the cholesterin and fats alone remain and rnay be collected on a filter and weighed.