The crystalline uric acid deposit is similarly often left after the salts are dissolved ; its solubility, on the addition of a considerable amount of nitric acid, distinguishes it from blood, and from fine sand, which hysterical patients occasionally mix with their urine. Its general characters are so marked that there is no real difficulty in recognizing it. Sometimes found in urine, which becomes perfectly transparent on standing, it is very frequently accompanied by a cloud of mucus; and it may be seen in other instances mixed with blood : in either case the urine will not become transparent when boiled with acid, although the uric acid itself may disappear.
Part of the sediment may be dissolved by heat or acid, but part may still remain, or it may have been wholly unaffected by either reagent. To a fresh portion of urine liquor potassie is added, and heat applied ; under all circum stances, a precipitation of the earthy salts takes place, and by careful boiling, these may be collected into a mass, which floats in the fluid, leaving the re mainder perfectly transparent, when the opacity has been caused by purulent deposit. If the quantity of pus be not sufficient to make the fluid ropy, a few drops of acid are allowed to fall on the aggregation of earthy salts, which are readily dissipated; and if the alkali have dissolved any pus, or other albumi nous principle, a very distinct cloud is formed in the urine, in consequence of the coagulation of albumen by the acid. The circumstance of a whitish de posit, which was not wholly soluble in heat and nitric acid, being partly dis solved by liquor potassze with heat, and the addition of nitric acid to this solution causing the remainder to disappear, and a cloud of albumen to form, may be received as pretty satisfactory evidence of the admixture of pus.
Nothing has been said of cystine, although it has nearly similar relations to chemical tests, simply because it is so rare. The fact of its being a fawn colored deposit, and yet not dissolving by heat alone, as similarly colored urates do, would necessarily lead to further inquiry ; purulent deposits are invariably white. Uric acid sand is also dissolved by boiling with an alkali ; but this can give rise to no confusion, because its characters are so striking.
While heating the urinary sediment, we have also to observe whether, as the temperature rises, the fluid first becomes transparent, and then a fresh cloud subsequently forms; or whether, when the opacity is not removed in the first instance, it becomes more tur bid, as the application of beat is continued ; and, further, what is the effect of one or two drops of acid upon this new precipitate.
b. Microscopical appearances.—It is especially important for the student to correct by the microscope the conclusions he has arrived at from chemical analysis: and in all cases of doubt, its aid is most valuable. Almost any portion of urine placed in the field of the microscope will present some minute objects floating about ; but it is better to let it rest some time, and then to take a drop from the bottom of the vessel: it will often happen that the microscope shows that there is a tolerably abundant sediment, when it is scarcely observed with the naked eye, because it differs so little from the fluid in its power of refracting the rays of light. The student need never perplex himself trying to make out shapeless objects, but should confine his attention to a few of the more distinct organic and crystalline fbrmations.
Among the organic bodies we observe 1. Blood-globules: these, it must be remembered, are not often found in their normal form, but variously altered by the fluid in which they float, being sometimes unusually flattened, but per haps still more frequently ruptured, and unequally distended, so as to assume a crescentic or a somewhat globular form.
2. Pus and mucus-globules; which can scarcely be distinguished from each other, and are most readily known by their relative numbers : a few solitary globules may be certainly regarded as mucus, a very large number as certainly pus: they are also in some measure distinguishable by the circumstance that pus globules are more decidedly granular, mucus smoother, and more uniform.
3. Epithelium, when present, would determine any doubtful globules to be mucus rather than pus. One or two scales may be seen in almost any specimen of urine, and they are only of con sequence when tolerably abundant, as indicating irritation of the bladder or urinary passages. Epithelium is generally found in large quantity in the urine of women, being derived from the uterus and vagina : the scales of this variety are much larger than those which come from the bladder.