The readiest method for the detection of albumen is certainly that of pouring from half a drachm to a drachm of nitric acid to the bottom of a precipitate glass containing the urine to be examined. This can easily be effected by allowing it to run down the side of the glass; and if the fluids do not intermingle, coagulation of the albumen takes place only at their line of junc tion, and may be observed even when exceedingly faint, by vary ing the position of the glass with reference to the light, and observing the effect of its transmission and reflection. The chief source of fallacy is the fact that nitric acid has the povier of pre cipitating an excess of urate of ammonia when held in solution by any unusual circumstance. A few experiments will better teach the different appearance of the two precipitates than any description ; but if any doubt remain, a small quantity of the urine may be boiled after only so much acid has been added as is necessary to produce the precipitation; the cloud will wholly disappear if it consists only of the lithates.
In employing heat as a test of the presence of albumen, it is to be borne in mind that heat develops chemical action, and may produce a precipitate of phosphate of lime, and that the coagula tion of the albumen may be prevented either by the urine being alkaline or by a single drop of strong acid being added to a neutral specimen. Both difficulties are best avoided by acidu lating the urine first with acetic acid ; and then, if the upper part of the urine in the test-tube be boiled, while the lower part is only gently heated, the contrast of opacity above and transpa rence below is often sufficient to indicate the presence of an ex ceedingly small quantity of albumen.
In the first of these two methods we have the advantage of being able to say with considerable confidence whether, when the urine is opaque, this is increased at the junction of the two fluids: in cases where there is only a trace of albumen, even if the urine be perfectly transparent, the haze produced by its coagulation is more easily perceived in this experiment than when boiled in a test-tube. In either case it is difficult to apply the farther test of heat, because the precipitate formed by the acid becomes less perceptible when transfused through the fluid, and it may. consequently, seem to disappear, with heat; and we are uncertain whether it be soluble or insoluble—whether orate f ammonia or albumen had been thrown down by the acid. Other i circumstances, however, serve to determine that the haze is not caused by precipitation of the orates : if a deposit have already spontaneously occurred or if the urine be naturally very acid without deposit, no addition of nitric acid will precipitate the urates from the clear urine : and if the urine be pale, and of low specific gravity, it is impossible by such means to render these salts insoluble in the excess of water which exists : in any of these cases we may therefore assume that the haze is albumen.
We must be on our guard in employing this test with opaque urine, lest the patient be at the time suffering from gonorrhoea and employing some of the resinous remedies, which are eliminated by the kidney, and are decomposed by nitric acid in the urine. This precipitate is also insoluble by beat, but may be distinguished from albumen by its amount being considerable, and yet no coagulation occurring when heat alone is used, or used in conjunction with one of the weaker acids. Occasionally it may be of service, when the urine is opaque, to boil it with alkali, as mentioned in speaking of the chemical re lations of pus (I 4, a 4); the whole of the fluid may thus be rendered trans parent, with the exception of the masa of phosphate floating np and dawn, and an opportunity afforded of ascertaining, by the addition of acid, what amount of precipitation occurs. All pus has a certain quantity of albumen necessarily associated with it ; but it is very much greater when the discharge is from the kidney than when it comes only from the bladder.
The heat•test for albumen is most certain when the urine is perfectly limpid, and decidedly acid. Opacity can only be removed by filtering; and the process is just sufficiently troublesome never to be practised. The nearest approxi mation to truth, in applying heat in such cases, is to compare the urine in the test-tube, atter being boiled with another portion of the same fluid in a second tube of the same size.
In using the stronger acids, as is commonly done when the urine is alkaline or neutral, we encounter some difficulties and sources of error. If only a drop of acid be added before the urine is boiled it may prevent the precipitation of the albumen altogether; if more be used, the albumen is apt to be coagulated, and it is no longer the heat-test, but the acid-test. If, again, the urine be boiled without acid, a precipitate of phosphate is likely to be formed if the urine be alkaline ; and though this may be removed by a drop or two of acid, the further addition of acid may not afford such distinct evidence of the pre sence of albumen when the temperature is raised to the boiling point, and it may have to stand some time before the precipitation can be observed.