ADVANCED PHYSIOLOGICAL CHEMISTRY 437 



Cholin is a strongly basic body, and may be considered as ammonium 

 hydrate, in which three of the hydrogen atoms are replaced by three 

 methyl radicles and the third by oxy-ethyl : 



f(CH,) 8 

 N.'CHo CH Q OH 



{OH: 



Lecithin sometimes exists in combination with proteid, the compound 

 being called Lecithin-albumin. 



Cholesterin. The chemical relationships of this body are not well 

 known, and consequently the reactions and tests are entirely empirical. 

 Besides those described in the elementary portion, the most important 

 is Salkowski's reaction. 



A few crystals of cholesterin are placed in a thoroughly dry test- 

 tube and are dissolved in a few c.c. of chloroform. An equal bulk of 

 concentrated sulphuric acid is now added, and the mixture is well 

 shaken for about a minute and then allowed to stand. The chloroform 

 swims to the top and is coloured, at first red and afterwards purple, and 

 the acid sinks to the bottom and takes on a green fluorescence. If some 

 of the chloroform solution be removed to another test-tube, and shaken 

 with a drop of water it becomes colourless, to be again coloured by 

 adding more sulphuric acid. 



CHAPTER V. 



MILK. 



THE conversion of caseinogen into casein takes place in two stages. 

 The first action of the rennet is to change the caseinogen into a body 

 called soluble casein ; this then combines with calcium salts to form an 

 insoluble compound called casemate of calcium. In order to study 

 the conditions necessary for the clotting of milk a solution of caseinogen 

 should be prepared by the following method (Ringer's) : 



300 c.c. of milk are mixed with an equal bulk of water, and 10 per 

 cent, acetic acid is added till all the caseinogen has been precipitated. 

 The precipitate is filtered off and thoroughly washed with distilled 

 water until the washings are no longer acid in reaction. It is then 

 removed from the filter paper, and ground up in a mortar with solid 

 calcium carbonate. The resulting paste is thrown into 500 c.c. of 

 water placed in a tall vessel, and the solution is allowed to stand for 

 several hours. The fat, which was contained in the precipitate, rises to 

 the surface, the calcium carbonate sinks to the bottom, and the 



