PROXIMATE PRINCIPLES IN GENERAL. 65 



It is to be kept constantly in view, in the examination of an 

 animal tissue or fluid, that the object of the operation is simply the 

 separation of its ingredients from each other, and not their decomposi- 

 tion or ultimate analysis. Only the simplest forms of chemical 

 manipulation should, therefore, be employed. The substance to 

 be examined should first be subjected to evaporation, in order to 

 extract and estimate its water. This evaporation must be conducted 

 at a heat not above 212 F., since a higher temperature would de- 

 stroy or alter some of the animal ingredients. Then, from the 

 dried residue, chloride of sodium, alkaline sulphates, carbonates, 

 and phosphates may be extracted with water. Coloring matters 

 may be separated by alcohol. Oils may be dissolved out by ether, 

 &c. &c. When a chemical decomposition is unavoidable, it must 

 be kept in sight and afterward corrected. Thus the glyko-cholate 

 of soda of the bile is separated from certain other ingredients by 

 precipitating it with acetate of lead, forming glyko-cholate of lead ; 

 but this is afterward decomposed, in its turn, by carbonate of soda, 

 reproducing the original glyko-cholate of soda. Sometimes it is 

 impossible to extract a proximate principle in an entirely unaltered 

 form. Thus the fibrin of the blood can be separated only by allow- 

 ing it to coagulate ; and once coagulated, it is permanently altered, 

 and can no longer present all its original characters of fluidity, &c., 

 as it existed beforehand in the blood. In such instances as this, 

 we can only make allowance for an unavoidable difficulty, and be 

 careful that the substance suffers no further alteration. By bearing 

 in mind the above considerations, we may form a tolerably correct 

 estimate of the nature and quantity of all of the proximate princi- 

 ples existing in the substance under examination. 



The manner in which the proximate principles are associated 

 together, so as to form the animal tissues, is deserving of notice. 

 In every animal solid and fluid, there is a considerable number of 

 proximate principles, which are present in certain proportions, and 

 which are so united with each other that the mixture presents a 

 homogeneous appearance. But this union is of a complicated cha- 

 racter ; and the presence of each ingredient depends, to a certain 

 extent, upon that of the others. Some of them, such as the alkaline 

 carbonates and phosphates, are in solution directly in the water. 

 Some, which are insoluble in water, are held in solution by the 

 presence of other soluble substances. Thus, phosphate of lime is 

 held in solution in the urine by the bi-phosphate of soda. In the 

 blood, it is dissolved by the albumen, which is itself fluid by union 

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