A. ANALYSIS OF ANIMAL FLUIDS.
When an animal fluid is presented for ana lysis, it is necessary in the first place to acquire a knowledge of the ingredients which enter into its composition in order to be able to decide upon the best method of ascertaining their proportions. The means of determining the nature and quritity of the organic con stituents will be first described, leaving the saline matters for a subsequent section.
1. For the organic constituents.
It is possible that all the principles just enumerated may occur together; this, however, will very rarely happen unless we have to examine the contents of the stomach, when a still more .heterogeneous mass may be pre sented to us.
Ilavina, where practicable, taken the specific vrravity Of the liquid in order to acquire an rdea of its degree of concentration, we place a portion under the microscope, and are thus enabled at once to decide upon the presence of blood globules, pus globules, fatty or oily matters in suspension, the debris of tissues, crystals of various substances, as uric acid, cholesterin, &c., and may then pass to tests purely chemical for its Qualitative analysis.
a. By allowing the liquid to stand at rest for a few hours, we at once determine the presence of fibrin, which coagulates and separates spon taneously, at the same time enveloping the red globules and suspended particles in its meshes.
b. The clear liquid is heated to boiling; if albumen be present it coagulates, unless the solution be alkaline, when the addition of a few drops of nitric acid causes an immediate curd ling-. A drop or two of acetic acid added to the original liquid, if it produce coagulation, shews the presence of casein. We need not seek for msein if the fluid shews an acid re action, as it is coagulated by free acids in general. If mucus be present, some ambiguity may arise from the action of acetic acid, as this re-agent causes the coagulation of the mucus furnished by the alimentary canal and its appendages. When present, however, in appreciable quan tity, mucus always communicates to the fluid a certain degree of ropiness which leads us to suspect its presence. A confirmatory test for casein under such circumstances consists in adding a few grains of milk sugar and a little washed rennet ; if the mixture be heated for an hour or MO to about 120°, the casein coagu lates completely.
c. Fatty matters and cholesterin are revealed by the microscope, and may be separated by evaporating the fluid to dryness, digesting the residue vvith ether and filtering; by sponta neous evaporation of the ethereal solution, they are left behind with their usual physical characters.
d. The presence of sugar is best discovered by mixing the suspected fluid with yeast and placing it in an inverted tube over mercury for twenty-four hours, at a temperature of from 70° to 80° F., making at the same time a comparative experiment with an equal bulk of the same yeast diluted to the same extent with pure water. We cannot by yeast determine with certainty the presence of sugar in a pro portion less than ahtli of the liquid employed. A much more delicate test, and one which fur nishes more immediate results, has lately been proposed by Trommer, founded upon the fact that organic bodies, to which free alkali has been added in excess, and especially solutions of grape and milk sugar, dissolve freshly pre cipitated oxide of copper; when the saccharine solutions are boiled they are decomposed, and sub-oxide of copper is deposited of a charac teristic reddish brown colour. To apply this test, add to the suspected liquid a few drops of solution of caustic potass, so as to render it distinctly alkaline, then a small quantity of a dilute solution of sulphate of copper, agita ting to dissolve the precipitate ; a liquid of a blue colour varying in intensity with the quan tity of copper held in solution is obtained. Apply heat, and if sugar is present an ochre yellow or red precipitate of sub-oxide takes place, as soon as ebullition begins. I have already mentioned that the presence of milk sugar has the same effect as grape or dia betic sugar ; other animal matters produce a similar change. It is, however, very deli cate in its indications; a negative result therefore may be considered as decisive of the absence of sugar. If a precipitate occur, the presumption that we are examining a saccha rine liquid ought always to be confirmed by recourse to the unequivocal expedient of fer mentation. It is easy to concentrate the fluid if the sugar is very small in quantity ; the only case in which any ambiguity can arise. Sugar of milk has never been found but in the secre tion froin which it derives its name.