MILK 31 



ture of 15 C. (60 R). At higher temperatures the 

 specific gravity or density is decreased, and at lower tem- 

 peratures it is increased. 



The specific gravity depends not only upon the total 

 quantity of solids contained in the milk, but also upon the 

 relative proportion in which the individual solids are 

 present, because the individual solids are of different spe- 

 cific gravity. Fat shows the greatest difference, being 

 much lighter than the other solids ; it is even lighter than 

 water. The solids not fat are all heavier than water, 

 the specific gravity of the salts being 4.12, lactose 1.666, 

 and proteids 1.346 (Richmond) . Therefore the removal 

 of fat, i.e., skimming, increases the specific gravity, and 

 the addition of skim milk has the same effect, while the 

 addition of water reduces the specific gravity. But the 

 specific gravity has such a wide normal variation that 

 it is possible to remove a small amount of fat from milk 

 with a normally low specific gravity without causing the 

 specific gravity to rise above the normal range, and, 

 conversely, a certain amount of water may be added to 

 milk with a normally high specific gravity without lower- 

 ing the specific gravity below the normal limit. How- 

 ever, in the first case the per cent, of fat will be decreased, 

 and in the second there will be a decrease in both the per 

 cent, of fat and of solids not fat. When the specific 

 gravity of milk is raised above the normal by skimming 

 it may be brought within the normal range by the addi- 

 tion of water, but the per cent, of fat and of solids not 

 fat will be decreased. Therefore, in examining market 

 milk to detect skimming or the addition of skimmed 

 milk or water, the per cent, of fat and of solids not fat 

 must always be considered in connection with the spe- 

 cific gravity. (For method of determining the specific 



