438 PRACTICAL PHYSIOLOGY 



intervening fluid contains the caseinogen in combination with calcium 

 as calcium caseinogenate, which is soluble in water (Osborne). 



Three samples of the opalescent solution are removed by means of a 

 pipette, and placed in three test-tubes labelled A, B, and C. To A are 

 added a few drops of rennin ; to E a few drops of a 0*5 per cent, 

 phosphoric acid and some rennin ; to G a few drops of a 0'2 per cent, 

 solution of calcium chloride and some rennin. 



The three test-tubes are placed in the water bath at 40 C., when 

 it will be noticed that coagulation occurs only in B and C, in 

 which, besides the ferment, soluble calcium salts are present. 1 In A, 

 although no visible change has taken place, the caseinogen has been 

 converted into the so-called soluble casein, and all that is necessary for 

 the production of clotting is the presence of calcium in solution. That 

 this is so can be demonstrated by boiling the solution A so as to 

 destroy the ferment, then cooling and adding a few drops of a 2 per 

 cent, solution of calcium chloride, when a clot will at once form. 



The Chemical Nature of Caseinogen. Caseinogen is of the nature 

 of a weak acid. It is insoluble in water, but, if a few drops of a weak 

 alkali be added, it at once dissolves because it forms a salt which is 

 soluble. Shake up some pure caseinogen (prepared as described on p. 

 186) in a test-tube with distilled water; it does not dissolve. Add 

 some powdered calcium carbonate and shake. The caseino'gen, being 

 acid in nature, expels a certain amount of the carbonic acid, and takes its 

 place, forming caseinogenate of calcium, which is soluble. That partial 

 solution of the casein has taken place can be proved by filtering. The 

 filtrate is opalescent, and gives a precipitate of caseinogen on adding a 

 weak acid. 



In another test-tube shake up some caseinogen with a weak alkali, 

 such as lime-water : the caseinogen dissolves, and an opalescent 

 solution is produced. If all the caseinogen has not dissolved, filter. 

 Now add to the opalescent solution sufficient litmus solution to colour 

 it faint blue, and then neutralise carefully with a 0*5 per cent, solution of 

 phosphoric acid. This combines with the excess of calcium hydrate to 

 form calcium phosphate, and the caseinogen becomes swollen up so that a 

 solution looking like skimmed milk is obtained, especially on slightly 

 warming the solution. Besides these reactions caseinogen salts are pre- 

 cipitated by the addition of weak acids or by neutral salts, used in the 

 same strengths as for globulins (p. 176). Their solutions are not 

 coagulated by boiling. 



Chemically caseinogen is of the nature of a Pseudo-nudein (p. 179), and 



>f~ 



1 The phosphoric acid added to B brings some of the Ca salts suspended in the 

 opalescent fluid into solution. 



