CH. IX] FILTRATION. 227 



etc.) another portion of the original material which has 

 been dried at a temperature not above 30° C. is extracted 

 (1) with cold water, (2) the residue from (1) with dilute 

 alkali, 1 — 2 p.c. soda (NaOH). 



If much oil or fatty matter is present this should be 

 first removed by agitation with benzene in the cold ; the 

 residue washed with ether and dried below 30° can then 

 be treated with (1) water, (2) dilute alkali. 



Extract 1. (Cold water) contains soluble proteids, 

 peptones and albumoses, amides, nitrates and nitrites, 

 ammonium compounds, etc. etc. 



Extract 2. (Dilute alkali) contains proteids insoluble 

 in water but soluble in dilute alkali. 



These extractions (and the agitation with cold benzene) 

 may be made with the apparatus used for the cold water 

 extracts of non-nitrogenous substances (see p. 225). 



Filtration. 



It is often very difficult to filter clear the extracts of 

 vegetable tissues ; the use of some of the special kinds of 

 paper made by Sleicher and Schiill will frequently give a 

 clear filtrate where ordinary filter-paper fails, but filtering 

 through asbestos is very effective in troublesome cases. 



For filtering with asbestos it is necessary to use a 

 filter-pump, and the most convenient method of working 

 is to use a perforated porcelain filter plate with a hardened 

 paper (such as no. 57-5 in Schleicher and SchiiU's cata- 

 logue), on to which the asbestos is poured whilst suction 

 is applied below, the asbestos having previously been 

 stirred into a cream with warm water; after the water 



15—2 



