30 SCIENCE AND PRACTICE OF DAIRYING. 



as a rule, are not taken into account in the quantitative analysis 

 of milk. Among these are nuclein and lecithin substances which 

 have been already mentioned as constituents of the caseous matter 

 and of the fat of milk urea, hypoxanthin, ammonia, citric acid, 

 cholesterin, sulphates, sulphocyanates, carbonic acid, oxygen and 

 nitrogen gas. Small quantities of substances derived from the food 

 of the cow, but which possess no nutritive properties, such as 

 colouring substances and odorous substances, are also found as 

 occasional constituents. Peptone does not belong to the normal 

 constituents of milk, and it is doubtful whether milk, in addition 

 to milk-sugar, contains any other carbohydrate of the dextrine class 

 in small quantity as has been asserted. F. J. Harz has recently 

 found in milk and in milk products such a body, and has named it 

 amyloid. 



A peculiar interest attaches to the discovery of citric acid in cows' 

 milk, made by Henkel and confirmed by Anton Scheibe. It is found 

 also in goats' and in human milk. The percentage of citric acid in 

 cows' milk varies considerably. This variation, however, does not depend 

 on the feeding of the cow. On an average, it amounts to *1 to -15 per 

 cent of the milk. From the researches of Scheibe it appears that citric 

 acid is a specific constituent of milk, since, like the organic constituents of 

 milk, it is not originally present in the milk-glands in this form. 



In condensed milk, viz. that condensed without the addition of sugar, 

 and in sterilized or preserved milk, concretions or bulky precipitates com- 

 monly occur, as Henkel has pointed out, which consist almost entirely of 

 pure calcium citrate. 



Cows' milk contains only about *007 per cent of urea. Milk fresh 

 from the udder always contains a certain quantity of gases, oxygen, 

 nitrogen, and carbonic acid the carbonic acid predominating. They may 

 amount to 6 per cent or more of the volume of the milk. S. M. Babcock 

 claims to have shown that milk contains j^^^ of a per cent of fibrin; 

 but this requires further confirmation. 



10. The Percentage Composition of Cows' Milk. Very consider- 

 able variations are to be found both in the specific gravity and in the 

 composition of milk drawn even from the same cow at different times 

 (morning, mid-day, and evening). In the whole day's milk, yielded 

 by a single cow on the same day, the variations are within narrow 

 limits. This is still more the case where the samples are representa- 

 tive of a quantity of milk, drawn at the same time; and still more 

 to a quantity of day's-milk from a number of cows (more than five). 



