In the application of heat and nitric acid, the student will find it advanta geous to manipulate with at least three distinct portions of urine. He should pour about an ounce into a precipitate glass, and then add cautiously nearly a drachm of strong acid ; this will sink to the bottom of the glass if poured down its side, and if any reaction take place between the two fluids it will be distinctly seen at their line of junction. He should then pour a small quantity of urine alone (half a drachm is quite enough, and better than a larger quan tity) into a test-tube, and boil over a spint-lamp, and then add one or two drops of acid. To a third quantity he should add a few drops of acid while cold, and then boil. In all cases in which heat is employed he should be care ful to add neither too mach nor too little acid ; a large quantity produces chemical decomposition, which may be perplexing, and a single drop of acid sometimes prevents the precipitation of the albumen, which it is our great object to accomplish by these means.
In the use of alkali a considerable quantity is generally needed, and mis takes are more likely to be made by boiling with too small than with too large a proportion ; its action ought always to be aided by the application of heat; there are scarcely any points which can be solved by its admixture with the urine at ordinary temperature.
In proceeding with the analysis of the urine we will direct our attention (1) to its degree of acidity, (2) its specific grivity, (3) its general appearance, (4) the chemical constitution and micro scopical appearance of its sediments, (5) to the effects produced on the clear fluid by various reagents.
§ 1. Acidity and Alkalescence.—The degree of acidity can only be guessed at by the change of color which test-paper undergoes: it differs in health at various periods of the day, and has been ascertained to become even alkalescent during digestion in per sons who appeared to be perfectly healthy. Excessive acidity, whether occurring only at certain periods in the twenty-four hours, or characterizing the whole quantity passed, indicates faulty assimilation ; it may be due either to the formation of an unusual quantity of lithic acid, or to acid generated in the stomach during digestion, which subsequently passes into the circulation, and then appears in the urine ; acidity due to the former cause is more constant, that produced by the latter more marked after any excess in eating or drinking.
Alkalescence depends upon two very distinct causes—defi ciency of acid, and decomposition : the former indicated by an excess of fixed alkali, the latter by the presence of free ammonia.
The smell is generally sufficient to discriminate these two con ditions, and any doubt may be removed by heating the test-paper after use. Deficiency of acid when it exists as a permanent con dition, either characterizing the whole of the urine voided through out the day, or at least recurring very frequently, is that to which the name of the phosphatic diathesis was once applied, because it is generally accompanied by a deposit of the earthy phosphates, which are insoluble in alkaline solutions ; it generally implies a low state of vitality, and more particularly nervous depression, resulting from exhaustion of the nervous system by meutal anxiety, spermatorrhcea, &c. Similar results are occasionally met with from transient circumstances ; a person whose urine is usu ally neutral or slightly acid will pass very alkaline urine for a short time at the commencement of digestion, when suffering from acid dyspepsia ; most remarkable examples of this may be seen during the fermentation of the food in cases of sarcina ven triculi. Another accidental cause is the ingestion of a large quantity of any of the vegetable salts which are decomposed in the system, e. g, the tartrate or citrate of potash or soda.
The presence of volatile alkali depends chiefly on decomposi tion ; and in by far the larger number of cases is due to imper fect emptying of the bladder, which causes the secretion of un healthy mucus or pus from its lining membrane ; the idea that deficient nervous energy in paraplegia was the direct cause of decomposition is now abandoned. In certain states the urine passes more rapidly into, decomposition than in others ; and it would appear that deficiency of acid, amounting only to its being slightly below the ordinary standard, along with excessive secre tion of mucus from the bladder, though not actually morbid, may excite this change very soon after the urine is evacuated; and the same effect may be produced by impurities in the vessel in which it is contained, a very small quantity of animal matter in a state of change speedly rendering the urine fetid. This condition is very different from that just adverted to, when inflammation of the bladder exists; in the one decomposition takes place after the urine has left the bladder, in the other the urine is ammoniacal when passed ; the one occurs in states closely analogous to those in which fixed alkali is commonly present, the other is restricted to the cases of local disease.