ORGANIC ANALYSIS. 



795 



As the alkaline chlorides are, moreover, not 

 completely fixed at a red heat, some loss is not 

 unfrequently sustained by a long continued 

 ignition. We may dispense with the employ- 

 ment of nitric acid and avoid loss from volati- 

 lization by shortening the period during which 

 a higli temperature is kept up, and simply char- 

 ring the mass at a low red heat so as to destroy 

 all organic matter; the black residuum is then 

 digested in water, by which the alkaline salts 

 dissolve, and after well washing, a little weak 

 nitric acid is employed to remove the earthy 

 compounds : charcoal alone remains undis- 

 solved. By evaporation of the solutions the 

 amount of saline matter is ascertained. 



Filtration may be performed on good white 

 unsized blotting paper cut of a size to drop 

 completely within the funnel ; before pouring 

 in the fluid to be cleared, the filter should be 

 moistened, if the solution be aqueous, with 

 water; if alcoholic or ethereal, with a few drops 

 of alcohol or ether, as the case may be. In 

 order to ascertain the quantity of dry residue, 

 the preferable plan is to employ two filters pre- 

 viously counterpoised one against the other, 

 and to insert one into the other ; the excess of 

 weight the inner filter shews when the filtra- 

 tion is complete, after both have been carefully 

 dried on the water-bath, will furnish the quan- 

 tity of solid matter. To render the filtration 

 more rapid, the apex of the outer filter may be 

 cut off so as to leave only one thickness of 

 paper at the point. Occasionally we may 

 weigh the filter itself and mark its weight in 

 pencil upon it ; the objection to this plan con- 

 sists in the difficulty of obtaining the paper 

 always at the same point of dryness. It should 

 be first dried at 212F., placed in a light covered 

 capsule over sulphuric acid, and allowed to 

 cool, then weighed whilst covered from the air. 

 The same care must be observed on again 

 weighing it with the precipitate. Of course 

 in washing an organic precipitate, the same 

 precautions will be required as when pro- 

 ceeding with the analysis of a mineral. The 

 stream of washing liquid should be speci- 

 ally directed to the edges of the filter, which 

 may be known to be sufficiently washed, when 

 by the evaporation of a drop of the liquid 

 which passes through on a slip of glass, no 

 perceptible stain is produced. 



Whenever decantation, or pouring off from 

 the sediment, can be substituted (as it very 

 frequently may in alcoholic and ethereal di- 

 gestions) for filtration, it is to be preferred, as 

 some loss is unavoidably occasioned by every 

 filtration ; whereas by decantation the preci- 

 pitate may be dried in the vessel, and if this 

 have been previously counterpoised we can 

 ascertain the weight of adhering matter with 

 great exactness. By this method the precipitate 

 may be as perfectly washed as when filtration 

 is adopted ; the fluid may be poured off very 

 close to the solid matter, which may again be 

 diffused through a fresh portion of washing 

 fluid, again allowed to subside, and the wash- 

 ing repealed as often as may be necessary. In 

 pouring a fluid from one vessel to another, loss 

 is avoided by moistening a glass rod in the 



liquid, bringing it into contact with the lip of the 

 glass or basin, and pouring the liquid down this 

 rod, which is not removed until the side of the 

 vessel is nearly restored to the vertical position; 

 by observing this precaution we escape the risk 

 of losing any portion from its running down 

 the outside of the vessel. The drops adhering 

 to the rod are washed into the rest of the 

 solution. 



The animal substances that we have to ex- 

 amine naturally arrange themselves into fluids 

 and solids, and as this division is convenient 

 in a practical point of view, I shall here adopt 

 it, beginning with those presented to us in a 

 fluid state. 



In order to fix some definite limit to our 

 enquiries, those principles only will here be 

 noticed, which, from the frequency of their 

 occurrence or their importance as constituents 

 or products of the living frame, are most likely 

 to be the special objects of attention to the 

 physician and pathologist. In this number 

 among the fluid products I shall enumerate 

 fibrin, albumen, casein, fatty matters, urea, 

 sugar, the uric, urobenzoic, lactic, and oxalic 

 acids, mentioning a few other compounds, as 

 ptyalin, sulphocyanic acid, &c. &c., when 

 describing the general plan of analysing the 

 secretions in which they most commonly 

 occur. 



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 quantity 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. 



Having, where practicable, taken the specific 

 gravity of the liquid in order to acquire an 

 idea 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 



