96 PRACTICAL PHYSIOLOGY. [XIIT 



saturaterl solution of MgSOj. Add distilled water, which in presence of the 

 MgS04 dissolves the caseinogen, which })asses through the filter and is col- 

 lected. From the solution of caseinogen in weak MgSO^ precipitate the 

 caseinogen by excess of acetic acid. To get the caseinogen quite pure it must 

 be redissolved in weak alkali or lime water, and precipitated and rcdissolved 

 several times. 



Tlie filtrate after proci})itation of caseinogen contains the lactalbumin, and 

 can be completely precipitated by saturation with sodium sulphate. It coagu- 

 lates between 70" and 80' C, and does not seem to be separated into several 

 proteids by fractional heat coagulation. 



The fluid contains lactose, salts, and serum-albumin. Filter. 



3. Separation of Caseinogen and Fat by Filtration. — Using a Bunsen's 

 pump, filter milk through a porous cell of porcelain. The particulate matters — 

 caseinogen and fat— remain behind, while a clear filtrate containing the other 

 substances passes through. The porous cell is left empty 

 and fitted with a caoutchouc cork with two glass tubes 

 tightly fitted into it. One tube is closed with a clij) (fig. 49), 

 and the other is attached to the pump Place the ])orous 

 cell in an outer vessel containing milk. On exhausting 

 the ])orous cell a clear watery fluid slowly passes through. 

 Test it for proteids and sugar. Notice the absence of fat 

 and caseinogen. 



4. Souring of Milk. — Place a small quantity 

 of milk in a vessel in a warm place for several 

 days, when it turns sour and curdles. It becomes 



Fio —Porous Cell ^^^^ — ^^^^ *^^^^ (Lessou IX. 10) — iiaving under- 

 for"* the Filtration gone the lactic acld fermentation, the lactose 

 of Milk. being split up by a micro-organism into lactic acid. 



5. Butter. — Place a little milk in a narrow, cylindrical, stoppered 

 bottle ; add half its volume of caustic soda and some ether, and shake 

 the mixture. Put the bottle in a water-batli at a low temperature ; 

 the milk loses its wjiite colour, and an ethereal solution of the fats 

 floats on the surface. On evaporating the ethereal solution, the 

 butter is left behind. 



6. Curdling of Milk. 



(a.) By an Acid. — Place some milk in a flask ; warm it to 40° 

 C, and add a few drops of acetic acid. The mass clots or curdles, 

 and separates into a solid curd (caseinogen and fat), and a clear 

 fluid, the whey, which contains the lacto.se. Filter. 



(h.) By Rennet Ferment.— Take 5 cc. of fresh milk in a test- 

 tube, heat it in a water-bath to 40° C, and add to it a small 

 quantity of extract of rennet, or an equal volume of a glycerin 

 extract of the gastric mucous membrane, which has been neutral- 

 ised with dilute sodic carbonate, and place the tube again in the 

 water-bath at 40° C. 



Observe that the whole mass curdles in a few minutes, so that 



