CONSTITUENTS OF THE YOLK. 357 



the removal of the fat and the ether, and are rinced as long 

 as the fluid which passes through the filter exhibits any 

 opalescence on being heated, there will remain a mass per- 

 fectly similar to the casein prepared according to the direc- 

 tions of Rochleder andBopp, (see vol. i., pp. 379, 380,) and which 

 contains in addition to the true casein some portion of albumen 

 which is very poor in salts. This albumen will be precipitated by 

 diluting the yolk-fluid with water, precisely as we observe in the 

 case of white of egg and blood-serum. This substance possesses 

 all the properties ascribed to casein (vol. i.,p. 374), as we find from 

 its behaviour towards acids and alkalies, and the alkaline, earthy, 

 and metallic salts. We would simply observe that this substance 

 dissolves even in very dilute solutions of hydrochlorate of 

 ammonia, chloride of sodium, sulphate of soda, &c., leaving only 

 a small residue (consisting of a little fat and of the investing 

 membranes of the yolk-globules,) which renders the fluid opales- 

 cent. Acetic acid renders this solution very turbid, and boiling 

 has a similar effect in a less degree. The substance separated by 

 boiling is the albumen, which had been precipitated by the dilution 

 of the yolk with water, and which has been again dissolved by the 

 hydrochlorate of ammonia, &c., at the same time with the casein. 

 All these concurring similarities between casein and the substance 

 of the yolk, would not, however, have led us to regard this substance 

 as casein, if it did not further possess the property so peculiarly 

 characteristic of casein, of being completely coagulated by rennet. 

 Thus, for instance, if rennet be added to this substance when 

 dissolved in an extremely dilute solution of hydrochlorate of 

 ammonia or soda, a dense casein-like coagulum will be formed in 

 about two or three hours, at a temperature of about 30 C. As the 

 sugar must have been entirely removed by washing, it cannot be 

 supposed that this substance can in any way have contributed by 

 its metamorphic action to the formation of this coagulum. One 

 hundred parts of this substance in the dry state yielded ,5*044 of 

 ash, which consisted almost exclusively of earthy phosphates and 

 carbonates. 



In characterising as casein this substance, which has hitherto 

 been considered to be of a special nature, we do so with the reser- 

 vation that this identity must be only conditionally accepted until 

 we have better means of establishing the presence of casein, more 

 especially as we know, on the one hand, that casein itself is 

 probably a mixture of several substances (containing here, as m 

 the milk, at all events the investing membranes of fat-cells), while, 



