212 QUANTITATIVE ANALYSIS. 



then ignite completely by heating more strongly. If the 

 complete oxidation to ash cannot be accomplished in this 

 way, extract the residue on the water-bath with water, 

 filter through a thin ashless filter ; on which the ash contain- 

 ing carbon is also brought, so far as this may be done without 

 difficulty. ftinse out the dish, and filter several times with 

 hot water (of course rinsing \vater is used for washing the 

 filter), and dry the filter with its contents. Dry the platinum 

 dish also. Place the filter with the ash containing carbon in 

 the platinum dish and ignite. The oxidation to ash now takes 

 place quickly. Let cool, transfer the aqueous extract with- 

 out loss to the platinum dish, evaporate to dryness, and 

 ignite the residue. Naturally we may also evaporate the 

 aqueous extract by itself. The soluble and the insoluble 

 salts are thus separately determined. 



3. To Determine the Amount of Nitrogen use 1 to 1.5 

 g. according to the amount of nitrogen present, which is 

 greater with a meat diet than with mixed food. The exact 

 determination of the weight of the faeces used is ascertained as 

 in the case of the determination of the amount of water. The 

 dust which remains sticking to the neck of the Kjeldahl flask is 

 rinsed into the flask with a little water, 15 cc. of sulphuric 

 acid and 0.5 g. of copper sulphate are added, and the flask is 

 heated, at first with a small flame until the contents have a 

 faint-blue or green color, etc. If the oxidation takes place 

 slowly, it may be hastened by the cautious addition of a little 

 finely pulverized permanganate. Thirty-five cubic centi- 

 meters of fifth-normal acid are placed in the receiver. This 

 is sufficient for almost all cases. With faBces very rich in 

 nitrogen use 20 cc. of half-normal acid (for the details of the 

 process see under Urine, page 187). 



4. Determination of the Ether Extract (Fat). Three 

 or four grams of the material (accurately weighed) are 

 extracted in a Soxhlet apparatus with ether, the ether extract 



