COMPOSITION AND PROPERTIES OF MILK 295 



body divided by that of an equal volume of water ; but owing 

 to the variation of the coefficients of expansion of different 

 substances, it is necessary to specify the temperature at which 

 the comparison is made. For scientific purposes, specific 

 gravities are usually taken at 4 C., but that of milk is usually 

 determined at 60 F., and compared with that of water at the 

 same temperature. If the specific gravity is taken at any 

 other temperature, a correction must be made. This can be 

 done, with sufficient accuracy for practical purposes, by adding 

 o'oooi to the specific gravity for each degree above 60 F., or 

 subtracting a similar amount for each degree below that 

 temperature. 



The specific gravity of the mixed milk of dairy herds 

 varies from about 1*030 to 1*033; DU ^ m the milk of single 

 cows it may vary from ro26 to 1*038, or, excluding abnormal 

 samples, from about 1*028 to 1*035. Hydrometers specially 

 graduated for the determination of specific gravity of milk are 

 called " lactometers." They are usually graduated for com- 

 parison with water as 1000, and only the significant figures are 

 given. A sample of milk having a specific gravity of 1*0315 

 would therefore show 31*5 lactometer degrees. 



The specific gravity of any sample of milk depends upon 

 two variable factors, viz. (i) the percentage of non-fatty solids, 

 which are heavier than water, and increase the density ; and 

 (2) the percentage of fat, which is lighter than water, and 

 tends to reduce the density. It is clear, therefore, that the 

 specific gravity, or lactometer reading, by itself affords no 

 information regarding the quality of the milk. The addition 

 of water to milk lowers the specific gravity, and the removal of 

 cream (fat) raises it. It is possible, by adding water and 

 removing cream simultaneously, to sophisticate the milk 

 without altering the specific gravity. 



It has been established that equal increments in the per- 

 centage of fat lower the specific gravity of the milk by equal 

 amounts ; also that the variation of the specific gravity due to 

 the non-fatty solids is equally regular, notwithstanding their 

 heterogeneous character. Separated milk, entirely devoid of 

 fat, but containing 8-5 per cent, of non-fatty solids, has a 



