78 SUBSTANCES SOLUBLE IN WATER. 



removing the lead by sulphuretted hydrogen and any glucose 

 that may be present by rapid fermentation. The residue may be 

 exhausted with boiling 90 per cent, alcohol and allowed to crystal- 

 lize in the cold. But an accurate result cannot be expected, since, 

 in addition to other errors, mannite may be produced in consider- 

 able quantity by the fermentation of cane-sugar. 1 For particulars 

 of some substances allied to mannite see 212. 



The method of examination for bitter principles, glucosides, and 

 alkaloids has been described in 58 to 69. (See also 165 

 et seq. ; 171.) 



EXAMINATION FOR ALBUMINOIDS SOLUBLE IN WATER, 

 AMMONIACAL SALTS AND NITRIC ACID. 



92. Extraction of Albuminoids. It has already been observed 

 in 74 that if the residue, after extraction with ether and alcohol, 

 be exhausted with water the estimation of albuminoids in the 

 aqueous extract thus prepared will generally give inaccurate 

 results. A fresh portion of material should therefore be directly 

 exhausted with water, or, if much fixed oil is present, the extrac- 

 tion with water may be preceded by treatment with petroleum 

 spirit. After having removed the fixed oil (if necessary) from 

 about 10 grams, the residue is dried at 40 C., macerated with 

 100 cc. of water, with frequent agitation, for 4 to 6 hours, and 

 filtered as described in 71. If thought desirable the maceration 

 may be conducted at a temperature not exceeding 35 to 40. 

 (Compare also 225 et seq.) 



Detection. With a portion of the filtrate qualitative experi- 

 ments should be made. Among the reagents used for the detec- 

 tion of albumen, iodine and mercuric nitrate (containing as little 

 free nitric acid as possible) may be mentioned ; the former colours 

 it brown, whilst the latter produces a yellow colour, changing, on 

 the addition of a trace of nitrous acid, to a splendid red. The 

 addition of caustic potash to albumen, previously moistened with 

 solution of sulphate of copper, is followed by the appearance of a 

 bluish-violet colour. If the amount of albumen- present be rather 

 small, these experiments may be made with the precipitates obr 

 tained by the addition of an acid to the aqueous solution ( 93). 



Microchemical. These reagents also serve for the microchemical 

 detection of albumen. The latter substance possesses, moreover, 



1 Archiv d. Pharm. xv. 47 (Journ. Chem. Soc. xxxviii. 100). 



