§ 4. Sediment8.—In a very large number of instances, by allow ing the urine to stand for some hours after it has cooled, a sedi ment is deposited, which sometimes can only be discovered by placing a drop from the bottom of the vessel under the micro scope, but is often readily perceptible. These sediments are partly formed of substances which are merely intermixed with the urine, and not dissolved in it ; partly of soluble materials, some of which are more freely dissolved in urine at die tempera ture of the body than at the temperature of the air, the excess being deposited as it cools; and partly of elements which are more soluble in one form of combination than another, and after the lapse of a few hours are slowly precipitated in e,onsequence of spontaneous chemical change. • They vary much in their general characthr and appearance, so that an experienced observer frequently can pronounce with accuracy as to their nature from mere inspection ; but it seems to me better to classify them ae,cording to their chemical relations and their microscopical characters; and as we proceed, any pecu liar appearances will be mentioned which they more commonly present to the naked eye. The supernatant fluid should be first carefully decanted off for separate analysis ; and if any part of it be perfectly transparent., this should be placed by itself, as the evidence of the presence of albumen in small quantity is so much more satisfactory in a transparent fluid.
a. Chemical relations.—A portion of the sedimentary urine is to be poured into a test-tube, taking care not to fill it to more than an inch in depth (students often puzzle themselves by using too large a quantity) heat is then applied, and as the temperatare rises we observe the 'following effects.
1. The deposit is entirely dissolved. It is wholly composed of the urates and chiefly of the urate of ammonia. This sediment presents a flocculent appearance, is sometimes quite white, but more generally it is colored of a brown, yellow, orange, or pinkish hue, and when the red tinge is distinct, it is scarcely necessary to use any test to ascertain its character; no other red-colored de posit has the same flocculent appearance : the urine is always acid.
2. It does not disappear with heat. One or two drops of acid are then to be added to the heated fluid : if it now dissolve, we know that it consisted of earthy salts, probably in combination with phosphoric acid. These are always white, but do not pre sent any appearance sufficient at once to distinguish them ; they generally abound in alkaline urine.
8. It is unaffected by hydrochloric acid, but dissolves on the free addition of nitric acid ; and it is also soluble in alkali. This deposit consists of uncombined uric acid : its general characters are very marked ; it is heavy, readily falling to the bottom of the tube after agitation, and presents to the naked eye the appearance of red sand; it can only be confounded with blood-globules, which sometimes have a similar sand-like character, but are not nearly so heavy, and are only found in urine partly opaque ; they are quite insoluble.
4. It does not disappear with heat and acid. To a fresh por tion add freely liquor potassfe, and boil ; the greater part is dis solved, and by gently prolonging the heat the undissolved portion collects into a mass, which floats in perfectly transparent fluid. Such a sediment is probably composed chiefly of pus, the undis solved portion being earthy salts, which are precipitated either in part or wholly after the alkali was added. When the amount of pus is considerable we shall find, on pouring the fluid out of the test-tube, that it has acquired an adhesive property ; it is called ropy because it clings in lengthened strings to the lip of the tube in place of dropping freely. This sediment cannot be distinguished by the naked eye from white lithates or phosphates ; the urine is seldom strongly acid, and does not become transparent when the deposit has fallen ; it shows a great tendency to become alkaline, and then the pus becomes adhesive, and is gradually converted into a ropy mass.
5. When part of the sediment dissolves with heat, the remain der must be tested in the same manner both with nitric acid and with liquor potassa3; and we may thus determine that there exists along with the urates an admixture either of earthy salt which is very often oxalate of lime, or of crystalline uric acid, of pus, or of blood.
Such are the general answers which chemistry affords as to the nature of sediments ; and they have been stated in broad outline because the more deli cate and intricate results are really unnecessary when we have the micro scope to appeal to in all cases of doubt ; and the few facts above referred to are the only ones of real importance in treatment. In a very large proportion of cases the sediment is composed of orates; and according to the present belief of chemists, the form of combination in which the uric acid exists is that of orate of ammonia. This class of salts is at once recognized by their ready solution by heat : and when the urine becomes perfectly transparent there can be no doubt as to their true character.
Not unfrequently when the earthy salts are present, some portion of the sediment dissolves by heat : this is due to an admixture of the urates; as soon as the temperature has reached the boiling-point, there can be no further solu tion by heat, and we add one or two drops of acid; the phosphates are dis solved with the utmost facility. Any acid may suffice, but it is well to re member that, of those in common use as tests, the hydrochloric has no action on uric acid, while the nitric dissolves it as well as the earthy salt ; at the same time it requires a much larger addition of acid to produce the one effect than the other. Oxalate of lime scarcely ever exists as a visible sediment, but it is also dissolved by the stronger acids. If it were desirable to determine by chemical reaction whether a scanty deposit, having the character of an earthy salt, were phosphate or oxalate, we should test one portion with acetic acid, which dissolves the former, and not the latter.