ANALYSIS OF THE YOLK. 365 - 



ascertain the amount of solid residue and of salts; the latter 

 must, however, only be determined by Rose's method, since by the % 

 application of a strong red heat for the purpose of incinerating, we 

 should unquestionably lose a portion of phosphoric acid. To 

 determine with any degree of accuracy its albuminous constituents, 

 the uninjured yolk must naturally be in the first place freed from 

 adhering white, by repeated but quick rincing. The fluid of 

 the yolk, which must now be allowed to escape from its mem- 

 brane and very readily dries, must be weighed and shaken with 

 ether as long as it continues to impart the faintest yellow tint to 

 that menstruum. The remaining watery albuminous fluid then 

 forms a white semi-solid mass ; this must be shaken with water, 

 which takes up a great part of the albuminous matter ; its quantity 

 is determined by boiling; the fluid which is poured away from the 

 coagulum, is evaporated, and the residue is extracted with alcohol; 

 and the quantities of the water-extract and of the alcohol-extract 

 are thus determined ; this, however, serves only as a check for the 

 other method, according to which the quantity of the extracts is 

 determined from the solid residue of the whole fluid. 



A large proportion, however, of the albuminous constituents of 

 the yolk, comprising the casein and some albumen devoid of alkali, 

 remains undissolved in water; this portion is now to be treated 

 with a dilute solution of hydrochlorate of ammonia, by which the - 

 yolk -granules which are visible under the microscope, the casein, 

 and the albumen that has been precipitated by water, are dissolved. 

 The solution of these substances is heated, which precipitates the 

 albumen ; and the casein-containing fluid, after the separation of 

 the albumen by filtration, is then carefully treated with dilute 

 acetic acid, which precipitates the casein ; or conversely, the casein 

 is first thrown down by acetic acid, and the albumen is subsequently 

 precipitated by boiling, after the neutralisation of the acetic acid by 

 ammonia. In order, however, to remove all protein-matters from 

 the fluid, it is necessary that the filtered fluid should be evapo- 

 rated to dryness,and the residue freed from hydrochlorate and acetate 

 of ammonia by rincing with cold water. It is only further neces- 

 sary to refer to the rule laid down in p. 223, that all albuminous 

 substances, after being thoroughly washed with water, should also 

 be treated with boiling spirit, in order to remove any adhering 

 matters which are insoluble in water, but soluble in alcohol. 

 This is especially necessary in the case of vitellin, since this sub- 

 stance, when obtained in the above mentioned manner, is seldom 

 altogether free from fat. 



