96 MANUAL OF PHYSIOLOGY. 



the degree of density of solution, and the other the degree of 

 opacity of the emulsion. 



I. To test the density, a specially graduated form of hydrometer 

 is generally used. This is graduated so as to indicate specific 

 gravities from 1042 to 1014. The former being the maximum 

 density of pure milk, the average being about 1030, and the latter 

 being about the density of pure milk when mixed with an equal 

 bulk of water. Every reduction of 3 in the specific gravity may 

 be said to correspond to about 10 per cent, of water. 



II. The degree of opacity is estimated by the amount of water 

 required to render a small quantity of milk sufficiently translucent 

 to allow a candle-flame to be seen through a layer of the mixture 

 one centimetre thick. One cubic centimetre of the milk (which 

 has been shown by the microscope and the iodine test not to con- 

 tain chalk or starch) is placed in a test-glass with flat parallel 

 sides, just one centimetre apart, and water is cautiously added 

 from a graduated pipette. The more water required the richer 

 the milk is in fat ; good fresh milk requires about 70 times its 

 bulk of water to become translucent. 



Another method employed for the same purpose consists in the 

 comparison of the color produced by a thin layer of milk in a 

 black cell with a previously prepared standard of grayish colors. 



The quantity of fat may also be estimated by placing the milk 

 in a tall graduated vessel for twenty-four hours, at the end of 

 which time it should show at least 10 per cent, of cream. 



Butter is made from milk, or better from cream, by breaking 

 by agitation the coating of proteid which before churning prevents 

 the oil globules from running together. It is almost completely 

 composed of fat, the larger globules having run together to form 

 the solid butter, which can be removed, leaving some small fat 

 globules with the proteids, milk sugar, lactic acid, and salts in 

 the water forming " buttermilk."* 



Cheese is another form of food made from milk by precipitating 

 the proteid either by lactic fermentation, or the addition of rennet 

 an extract of calves' stomach which, without the presence of 



* For the details of secretion of milk, etc., see Mammary Gland. 



