§ 226. EXTRACTION WITH DILUTE ALKALI. 235 



repeated once more. The legumin can then be precipitated as 

 described. 1 



During all these manipulations, the exclusion as far as possible of 

 the carbonic acid in the air is most strongly to be urged. It would be 

 liable, in most cases, to cause an error by partially or completely pre- 

 cipitating albuminoids allied to (perhaps identical with) legumin, 

 such as globulin, or vegetable vitellin. 



If the material has not been previously entirely freed from fixed 

 oil (and this is not always possible with petroleum spirit) the 

 precipitated legumin must be finally washed with absolute alcohol 

 and with ether, to remove the oil mechanically retained by it. 



§ 226. Extraction with DUwte Alkali — Eitthausen has observed 

 that the strongly acid reaction of many freshly powdered seeds 

 decreases on keeping. Now strongly acid liquids are less suitable 

 for the extraction of legumin than neutral or alkaline ; Weyl,^ 

 moreover, asserts that fresh seeds contain no legumin, and' ex. 

 presses it as his opinion that this substance is produced from 

 vitelhn and myosin during the manipulatioiis. The diflFerences 

 that can thus arise are extremely inconvenient. It must also be 

 observed that pure water frequently extracts but very small 

 quantities of legumin from vegetable substances containing that 

 body, and that the major part is removed only by dilute alkali. 

 This portion of &e legumin (casein,^ glutencase'in, conglutin, 

 vegetable fibrin) will therefore be dissolved when the residue in- 

 soluble in water is treated with dilute (0-1 to 0-2 per cent.) soda 

 or potash (§§ 103 to 106). If the presence of legumin is suspected 

 it is advisable to extract at a temperature of 4° to 6°. (See also 

 § 233.) 



Should metaraMc acid (§ 1 95) and legumin be extracted together, 

 a total estimation must first be made, and the nitrogen then de- 

 ternuned in a part of the precipitate, from which the amount of 

 legumin present can be calculated. 



A current of carbonic acid might be passed through the alkaline 

 extract to ascertain whether globulin (viteUin, myosin) can be thus 

 precipitated (§ 93) ; should this prove to be the case, it might be 



' Compare Kitthausen, loc. cit. 144. 



2 Beitr. z. Kenntnisa d. thier. und pflanz. Eiweisskorper. Diss. 1877. 



* The substances here referred to agree in most of their properties, but 

 exhibit certain differences ; thus, glutencasein contains more sulphur, and is 

 more easily soluble in acetic acid than legumin. Ritthaiisen advocates the 

 use of the word ' casein ' as a group-designation. 



