328 SCIENCE AND PRACTICE OF DAIRYING. 



gravity of milk and skim-milk (observed at temperatures from 

 and 30 C.), to 15 C. All comparisons are made at that tempera- 

 ture, for the sake of simplicity in practice. When the specific gravity 

 of milk is stated, the first two figures, along with the point, are 

 removed. Thus, for example, a sample of milk having a specific 

 gravity of 1*03175 at 15 C., is spoken of as having a specific gravity 

 of 3175. 



For example, if the specific gravity of milk at 24 C. has been 

 found to be 2970; at 15 C., therefore, it will be 31-2 + -1 X 7, equal 

 to 31'9. There is found on Table II. the numbers 31'2 and 32'2 

 for 29 and 30 respectively, at 24 C. The difference for one degree 

 amounts to 1, for a tenth of a degree *1, and for seven-tenths 7. 



The specific gravity of milk may be stated in different ways. 

 It may be stated in comparison to distilled water at 15 C., and 

 weighed in air, or it may be stated in comparison with water at 4 

 C., and weighed in air or water at 4 C., and calculated in vacuum. 

 According to the method of comparison, the numbers will naturally 

 differ. If, for example, the specific gravity of a sample of milk 

 has been determined by the pyknometer at 15 C. and compared 

 with distilled water at the same temperature, and weighed in air, 

 and found to be T0315, and if it be desired to convert this number 

 into comparison with water at 4 C., taking the density of water at 

 15 C. at -99916, then the figure will be found by multiplying 

 1-0315 by '99916, that is, T03063. The difference amounts to 

 1-0315 -1-03063 = -00087. If it be desired to calculate this in 

 vacuum, it will be found by multiplying T0315 into ('99916 

 00119) + -00119, that is, T03060. The figures, then, for specific 

 gravities are as follows: 



Weighed in air and compared with water at 15 C., equal to 1-03150. 



4C, 1-03063. 

 in vacuum 4 C.j 1-03060. 



As it is sufficient for practical and scientific purposes to know 

 the specific gravity to four places of decimals, it will make little 

 difference whether it is calculated to water at 4 C., or whether it is 

 weighed in vacuum or not. On the other hand, it is not the same 

 whether the specific gravity be taken with reference to water at 

 15 C. or at 4 C. As a rule, the specific gravity of milk is calcu- 

 lated at 15 C., and compared with distilled water at the same 

 temperature. 



