34 



milk to curdle. The coagulum or curd, incloses the fat 

 globules. The liquid from which it is separated, a solution 

 of milk, sugar and salts, is known as whey. The curd, 

 when completely separated from the whey, is called cheese. 



146. Test the reaction of milk. Fresh cow's milk may 

 often be neutral or even acid. Sour milk is acid. The re- 

 action of fresh human milk is always alkaline. Free lactic 

 acid is present in the fresh milk of the carnivora. 



147. Fill a cylindrical glass jar two-thirds full of unskim- 

 med milk. Insert the hydrometer and take the specific 

 gravity. Repeat the determination with the New York 

 board of health lactometer. This instrument is so con- 

 structed that 100 at 60° F. represents a specific gravity 

 1.029, below which unadulterated milk is supposed never to 

 fall. The laws of New York require milk to have a density 

 of not less than 1.029, ^^'^ total solids of not less than 12%, 

 of which 3% must be fats. 



148. Examine a drop of fresh cow's milk under the micro- 

 scope. It consists of a clear fluid containing a large number 

 of highly refractive fat globules. lyct a drop of osmic acid 

 solution run under the cover glass ; in a short time the 

 globules become stained brown- black. 



149. Mix 5 cc. of fresh milk with 15 drops of neutral ar- 

 tificial gastric juice, and heat in the water bath to 40° C. In 

 a short time the milk curdles so that the tube can be inverted 

 without the curd falling out. By and by the whey is squeezed 

 out of the clot. The curdling of milk by the rennet ferment 

 present in the gastric juice is quite different from that pro- 

 duced by the " souring of milk," or by the precipitation of 

 caseinogen by acids. Here the casein (carrying with it most 

 of the fats) is precipitated in a neutral fluid. 



150. To the same test-tube after the above process add 10 

 cc. of 0.2% hydrochloric acid, and put into the incubator 

 until the next exercise. Note any changes when next 

 examined. 



