254 § 131. FODDER AND FOOD. 



solution, for the water dissolves some of the glass with 

 which it comes in contact. 



If it is not desired to subject the residue insoluble in 

 water to further treatment with alcohol and ether, as 

 below, it can be dried and weighed, and ash and nitrogen 

 determinations made with portions of it ; then, by sub- 

 tracting the amount of ash and nitrogen found in it from 

 what was found in the original substance, the amount that 

 should be found in the aqueous extract may be estimated, 

 and thus the tedious evaporation of a portion of that ' 

 extract to dryness be avoided; or the two sets of deter- 

 minations may be made to control each other ; the differ- 

 ence between the amount of substance taken for extraction 

 with water and the weight of the insoluble substance, 

 above determined, will give the amount soluble in water. 



Or, since the alcohol and ether used for extracting the 

 residue insoluble in water, as below, usually take up but 

 traces of mineral matters, we can, in case these solvents 

 take but a few per cent of that residue into solution, 

 consider the residue insoluble in alcohol and ether as con- 

 taining the same amount of ash ingredients as the residue 

 insoluble in water only, and the determination of the 

 ash in this will answer the same purpose as if estimated 

 in the first insoluble residue. 



c. Nitrogen in Forms Soluble in Water. — Evaporate 

 500-1000 c.c. of the aqueous extract to a syrup, on the 

 water-bath, absorb the residue by as small a quantity of 

 calcined gypsum as possible, collect the whole in a watch- 

 glass, dry it for a while at 88-90° C, and ignite the residue 

 with soda-lime (§ 85, a). 



d. Actual Ammonia* — Detennine this in a portion of 

 the aqueous extract, by Schlossing's method. Acidify 

 300 c.c. of the extract with hydrochloric acid, and con- 

 centrate it. A more accurate result may be obtained by 

 Knop's method of setting free the nitrogen and measuring 



