In one or two instances I have seemed to get more precise results in doubt ftil cases by warming the urine, so as to prevent the precipitation of the urates, without coagulating the albumen, and then applying the nitric acid test in a precipitate glass, as already mentioned.
When the results of the examination are at all unsatisfactory, very explicit directions should be given to the patient to preserve any portion of the urine that is transparent when passed, in a perfectly clean vessel : not uncommonly the difficulties are caused solely by some accidental admixture, and in all circumstances, a second analysis may throw much light upon the previous one. In other instances, when the trace of albumen is but slight, it is very necessary for correct diagnosis to make a second or a third examination, at intervals, because the casual occurrence of albu men in small quantity is generally not a matter of very great importance, and if urine passed at other times be distinctly free from it, there is every probability that it is not caused by organic change. Further, it may be added that, when any suspicion occurs to the mind of the possible existence of albuminuria, one examination alone, however exactly performed, and however ex plicit in its results, is not sufficient, because, as the casual pre,sence of albumen is no certain proof of the existence of organic change, so the urine may be casually free from it, in any stage of disease of the kidney.
b. Sugar.—In considering the general sytnptoms of disease in their relation to the urinary organs, in the succeeding chapter, the aggregate of symptoms in a case of diabetes will be found of such a character as to mark out very clearly the nature of the disease ; yet it is needful to be able to pronounce positively in any given case whether sugar be present in the urine. Ia the greater number of cases, the changes produced, when diabetic urine is boiled with an equal quantity of liquor potassre, are quite sufficient for the purpose. The urine becomes gradually of a deep yellow, which passes into brown, and then assumes a crim son or ruby appe,arance by transmitted light, exhaling an odor of burnt sugar or caramel. This sequence of changes cannot be misinterpreted by any one who has performed the experiment two or three times ; but it is open to fallacy if one who is not familiar with the test trusts to it alone, and it may fail to detect a very minute quantity of sugar.
Greater certainty can be attained by the action of sugar upon salts of copper, and students ought on all occasions to familiarize themselves with it A few drops of a strong solution of sulphate of copper are added to the urine, and then a considerable quantity of liquor potassa3: the first portion of the alkali causes precipita tion, its further addition dissolves the precipitate so formed: heat is now applied ; and, when the temperature rises to a certain point, a yellow precipitate is rapidly formed. The accuracy of
the test depends upon the two circumstance,s coinciding, that the precipitate has an evident tinge of yellow, and that it is formed with rapidity. A variety of chemical changes may precipitate the copper on prolonged boiling, but its color is usually tawny or brown ; and practically it is found that the only condition which gives rise to the rapid formation of a yellow precipitate is the presence of diabetic or grape-sugar.
The specific gravity of diabetic urine is invariably high, although the quantity passed be much greater than in health and while it is true that the absolute amount of the sugar dis solved in the fluid is one of the causes of its increased density, it is not the only one, and therefore the specific gravity cannot be taken as a measure of the saccharine matter present. In no other condition of disease is the density so great; and yet there is rarely any deposit, the urine being generally pale, straw-colored, and very often having a sort of oiliness in being poured from one vessel to another.
c. Urea.—When the specific gravity of the clear urine is above the normal standard, whether there be any deposit or not, and we have ascertained that no sugar is present, the conclusion is un avoidable that it is impregnated with an unusually large amount of those soluble principles which give to urine its ordinary cha racteristic properties, derived from metamorphosis of tissue; the most important of which is urea. The uric acid salts we. have seen may be held in solution by an excess of alkali, and are pre cipitated by nitric acid ; urea is not so precipitated ; if no change occur on the addition of a small quantity of acid, a little urine may be poured into a flat glass (a watch-glass), and about half its bulk of strong acid added to the fluid, when, if urea be present in considerable excess, feathery crystals of nitrate of urea will form. To produce this effect, its amount must be verb considerable, so that even when we do not obtain it, we are not justified in assert ing that no excess of urea is present : when the urine is deep colored, its density great, and its peculiar odor well marked, there can be no doubt of the fact that those principles among which urea holds a first place, are secreted in large quantity, and treat ment must be guided by this assurance.
Occasionally the urea is converted into carbonate of ammonia by some catalytic action, which probably commences before the urine is passed, but is greatly promoted by the action of heat. When the urine is boiled in such a case, the amount of earthy salts thrown down is generally considerable, and on the addition of acid the precipitate is dissolved, while effervescence takes place from decomposition of the carbonate of ammonia. This action cannot be regarded as any evidence of an excess of urea.