438 THE GLANDULAR ORGANS. 



of the neutral fats, the butter is first saponified, when the resulting 

 soaps may be separated from each other according to the usual 

 methods of analysis. 



Quantitative Estimation. The amount of fat in milk is most 

 conveniently estimated densimetrically with Soxhlet's apparatus. 

 To this end, a known amount of milk is mixed with a solution 

 of sodium hydrate and the fat extracted with a definite quantity 

 of ether. The ethereal solution is allowed to separate, and is 

 then forced into a glass cylinder provided with an aerometer. From 

 the specific gravity the percentage of fat is then read off from a table 

 which accompanies the apparatus. The latter is so constructed that 

 evaporation of the ether cannot occur. 



In the absence of such an apparatus the amount of fat can be 

 ascertained gravimetrically as follows : 20 c.c. of milk are treated 

 with a small amount of sodium hydrate solution, and are extracted 

 with 80 c.c. of ether which has been saturated with water. This is 

 done by shaking in a tightly closed bottle. After the ethereal ex- 

 tract has entirely separated, 60 c.c. are placed in a weighed beaker ; 

 the ether is allowed to evaporate ; the residue is dried and weighed. 

 The result is calculated out for 80 c.c. of the ethereal extract, 

 corresponding to 20 c.c. of milk. 



Lactose. In the animal body lactose is found only in the milk, if 

 we disregard the small amounts that may appear in the urine of nurs- 

 ing females, and which must hence of necessity occur also in the 

 blood. It is formed in the mammary glands, and may possibly 

 be related to the reducing substance which results from the nucleo- 

 glucoproteid when this is boiled with mineral acids. On exposure 

 to the air it undergoes a peculiar fermentation, with the formation 

 of lactic acid. This, in turn, combines with the calcium of the 

 lime-casein, and as a result the casein separates out, and constitutes 

 what is popularly termed clabber. On further standing, this con- 

 tracts, and finally floats in a clear, light-yellow fluid the add whey. 

 The fermentation in question is produced by definite micro-organ- 

 isms, of which fourteen varieties are now known. 



On inversion, lactose is decomposed into glucose and galactose. 



Isolation. To isolate lactose from milk, this is first curdled by 

 the addition of chymosin. The filtrate is slightly acidified with 

 acetic acid and boiled, so as to remove the coagulable albumins. The 

 second filtrate is then concentrated to a small volume, when on cool- 

 ing the lactose crystallizes out. To purify the substance, this is dis- 

 solved in water, decolorized with animal charcoal, and recrystallized 

 by evaporation. It is thus obtained in the form of white rhombic 

 prisms, which are soluble in water, but insoluble in absolute alcohol. 

 The substance has a somewhat sweetish taste, and contains one mole- 

 cule of water of crystallization, which rapidly escapes at 130 C. 



Estimation. To estimate the amount of lactose, the milk must 



