52 



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

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

 of highly retractive fat globules. Let a drop of osmic acid 

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

 become stained brown-black. 



177. To some milk in a test tube add a few drops of 

 sodium or potassium hydrate and heat. The liquid becomes 

 yellow, then orange, and finally brown. 



178. To a 4% solution of lactose add some of the same 

 reagent and heat. The same color is developed as in 177, 

 which is due to the sugar present in the milk. 



179. To some fresh milk in a test tube add a few drops 

 of fresh tincture of guaiac (20% solution in alcohol); agitate 

 and add some peroxide of hydrogen (or old terpentine). A 

 blue color develops. A similar result is given by the blood. 



180. Repeat the experiment with milk that has been 

 boiled. The blue color is not given — due to changes in the 

 proteid. 



181. 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 rennin engyme 

 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. 



182. 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 ex- 

 amined and test for peptones. 



183. Dilute 5 cc. of milk with 15 cc. of water, add a little 

 dilute acetic acid and warm. A precipitate is formed. Fil- 

 ter and save both precipitate and filtrate. This precipitate 

 is not the same as that obtained by rennet. The acid pre- 

 cipitate is caseinogen, and is freely soluble in dilute alkali, 

 the rennet clot is " casein," and is much less soluble in dilute 

 alkali. Cheese is made with rennin and cannot be made with 

 acid. 



