70 CHEMICAL PHYSIOLOGY. 



the numerous small, highly-refractive fat globules floating in 

 a fluid (Fig. 12, M). 



(i.) Add a drop of acetic acid, and observe how the 

 globules run into groups. 



(ii.) To a fresh 

 drop add osmic 

 acid, and observe 

 how the globules 

 first become brown 

 and then black. 



(iii.) If a drop of 

 colostrum is ob- 

 tainable, examine 

 for the " colostrum 

 corpuscles " (Fig. 

 12, 0). 



(6.) Examine the 

 " naked-eye " charac- 

 ters of milk. 



(') Test its reac- 

 tion with litmus- 

 paper. It is usually alkaline. 



(d.) Take the specific gravity of perfectly fresh unskimmed 

 milk with a hydrometer or lactometer. It is usually be- 

 tween 1025 - 1030. Take the specific gravity next day 

 after the cream has risen to the surface. The specific gravity 

 is higher. 



(e.) Dilute milk with ten times its volume of water, and 

 carefully neutralise it with dilute acetic acid, observe that 

 there is no precipitate, as the casein is prevented from 

 being precipitated by the presence of alkaline phosphates 

 (Lesson I., 7). Cautiously add the acetic acid until there 

 is a copious granular-looking precipitate of casein, which as 

 it falls, entangles the greater part of the fat in it. If desired 

 the precipitation is hastened by heating the fluid to 70 C. 



(/.) Filter (e.) through a moist-plaited filter. Keep the 

 residue on the filter. The filtrate of (/) should be clear. 

 Divide it into two portions. Take one of the portions,, 



Fig. 12. Microscopic appearance of milk. 



