THE ALBUMINOIDS 165 



Note that the magenta soon begins to pass through the parch- 

 ment into the water, but the cochineal diffuses nmch less 

 readily. Now repeat the experiment with starch mucilage and 

 solution of sugar, and after a short time test the water in each 

 beaker by appropriate tests for starch and sugar. It will be 

 found that, as in the previous case, it is the crystallisable sub- 

 stance, or crystalloid^ that is readily capable of diffusion, while 

 the non-crystallisable substance, or colloid^ does not easily 

 diffuse. Now try the experiment with a solution of albumin 

 before and after peptonisation, testing the water for diffused 

 substance by evaporating a portion to dryness. Note that the 

 unchanged albumin is colloid ; the peptone is crystalloid. 



This process obviously provides a method of separating crystalloids from 

 colloids in solution. The method is known as dialysis. In the foregoing 

 experiments it will be found that liquid passes through the diaphragm 

 in each direction ; and, in the cases of difficultly diffusible substances, 

 more liquid passes into the funnel than passes out. This gives rise to 

 increase of volume in the funnel, and consequent increase of pressuire. It 

 is this osmotic pressure to which, in the cells of plants, is largely due the 

 flow of sap. 



Milk. — I. Composition. — A drop of milk examined under a microscope 

 is seen to be an " emulsion," and to consist chiefly of minute globules of fat 

 floating in an aqueous liquid. The fat has a lower specific gravity than the 

 aqueous portion, and therefore separates as "cream" on standing, or on 

 submitting the milk to a rotatory movement in a " separator," when the 

 greater centrifugal force of the heavier liquid causes its separation from the 

 fat globules. This watery liquid, or "skim milk," contains the dissolved 

 albuminoids, and of these the casein coagulates on the addition of rennet to 

 the milk, a curd being produced, which, when compressed into a solid 

 mass, and then ripened by keeping, constitutes " cheese." (Cheeses contain 

 24 to 41 per cent, of water, 23 to 39 per cent, of fat, depending on the 

 proportion of cream in the milk used, 27 to 32 per cent, of casein, and 

 0.7 to 4.4 per cent, of salt.) 



The "whey" from the curd still contains the albumin of milk, and this 

 may now be coagulated by heating. If the precipitate be filtered off, and 

 the filtrate evaporated to a low bulk, crystals of the sugar of milk are 

 obtained. If the mother-liquor from these crystals be evaporated to dry- 



