METHODS OF ANALYSIS. 225 



is best accomplished by carbonising the solid residue of each, 

 and then adopting the modification of Rose's method, which is 

 described in p. 407 of the first volume. The salts can then be 

 analysed in accordance with the rules recently laid down by Rose, 

 whose labours have gone far to bring this department of analytical 

 chemistry to a state of perfection. 



The quantitative determination of the/ate in the blood, as in 

 other animal substances, is associated with difficulties which often 

 cannot be entirely overcome. As a matter of course, we only 

 employ for this purpose the solid residue, after thorough drying at 

 J20. The best method of procedure is to introduce into a small 

 digesting flask the dry substance which is to be employed for the 

 determination of the fat, while we determine its weight, as in 

 elementary analyses, by re-weighing. A small digesting flask is 

 necessary for this purpose, since it is only thus that we can boil 

 the substance with ether, and pour off the ethereal fatty solution 

 without loss of fat. The ethereal solution is then to be evaporated 

 from a small glass cup or basin with a very high border, because 

 the fat very readily creeps up to the edges, and thus necessarily 

 occasions loss. Moreover, the ether must be perfectly pure, as 

 free as possible from water, alcohol, and free acid. The evapo- 

 ration of the ether must be accomplished without boiling; and the 

 fatty residue, like all other residues, be dried at 120. 



Pure ether extracts only the neutral fats and the free fatty 

 acids, and riot the alkaline sails of the fatty acids; the latter must 

 be extracted with absolute alcohol, to which about one-tenth of its 

 volume of ether has been added. The determination of the soaps 

 is always uncertain, because, as a general rule, we do not obtain 

 them in sufficiently large quantities to separate the non-fatty 

 substances which almost invariably intermingle with them. 



To calculate the extractive matters by deducting from 100 

 parts of the fluid the sum of the constituents obtained by direct 

 analysis, is a procedure by no means to be recommended ; for by 

 such a course we lose one of the most important means of checking 

 or controlling the whole analysis. After the removal of the fats 

 from the solid residue which we are going to analyse, the extractive 

 matters, that is to say, the alcoholic, spirituous, and watery 

 extracts, must be dried, weighed, and finally incinerated, in order 

 that the ashes (after their determination) may be abstracted from 

 the organic matter ; it is only in this manner that we can hope to 

 attach any scientific value to these extractive matters, which, in a 

 physiological point of view, are doubtless of much importance. 



VOL. II. Q 



