272 EXCRETION. 



evaporated, the residue is extracted with boiling alcohol, in 

 the proportion of about a fluidrachm for every hundred 

 grains of the original weight of the specimen, filtered while 

 hot into a watch-glass, and allowed to evaporate spontane- 

 ously. To keep the fluid hot while filtering, the whole appa- 

 ratus may be placed in the chamber of a large water-bath, 

 or, as the filtration is generally rapid, the funnel may be 

 warmed by plunging it into hot water, or steaming it, taking 

 care that it be carefully wiped. We now have the choleste- 

 rine mixed with a certain quantity of saponifiable fat. After 

 the fluid has evaporated, we can see the cholesterine crystal- 

 lized in the watch-glass, mingled with masses of fat. This 

 we remove by saponification with an alkali ; and for 'this 

 purpose, we add a moderately strong solution of caustic pot- 

 ash, which we allow to remain in contact with the residue 

 for from one to two hours. If much fat be present, it is best 

 to heat the mixture to a temperature a little below the boil- 

 ing point ; but in analyses of the blood this is not necessary. 

 The mixture is then to be largely diluted with distilled water, 

 thrown upon a small filter, and thoroughly washed till the 

 fluid which passes through is neutral. "We then dry the 

 filter, and fill it up with ether, which, in passing through, 

 dissolves out the cholesterine. The ether is then evaporated, 

 the residue extracted with boiling alcohol, as before, the 

 alcohol collected on a watch-glass previously weighed, and 

 allowed to evaporate. The residue consists of pure choleste- 

 rine, the quantity of which may be estimated by weight. 



The accuracy of this process may be tested by means of 

 the microscope ; for the crystals have so distinctive a form, 

 that it is easy to determine, by examining the watch-glass, 

 whether the cholesterine be perfectly pure. In making this 

 analysis quantitatively, it is necessary to be very careful in 

 all the manipulations ; and for determining the weight of 

 such minute quantities, an accurate and delicate balance, 

 one, at least, that will turn with the thousandth of a gramme, 

 carefully adjusted, must be employed. With these precau- 



