380 THE CHEMICAL CONTROL OF THE DAIRY. 



of series (1). This may be due to two causes either the milk 

 contains more cream than the original, or it has been watered. 

 In some instances both causes may be responsible for the lowness 

 of the specific gravity. 



If the second or third cases have occurred a further examina- 

 tion of the milk should be proceeded with. Either the fat 

 or total solids should be estimated, and the solids not fat 

 calculated ; the corresponding sample of series (1) should be 

 examined simultaneously. It is advisable that the samples 

 or a selected number of the'm should be examined if it has 

 been found that the specific gravities agree, as it affords a means 

 of checking the accuracy of the specific gravity determinations, 

 and of detecting the somewhat hypothetical case of scientific 

 skimming and watering. 



A very important rule, which is of great use in the control 

 work of the dairy chemist, may be enunciated as follows : 



The specific gravity of a milk in lactometer degrees added 

 to the percentage of fat \vill remain practically constant, whether 

 the cream has been diminished or increased. The sum of the 

 two will be lowered by the addition of water. For instance, 

 the original milk had a specific gravity of 1-0325 or 32-5, and 

 contained 4 per cent, of fat. The sum is, therefore, 36-5. 



If a sample of (say) series (2) were found to have a specific 

 gravity of 1-0315 or 31-5, and contained 5-0 per cent, of fat, the 

 sum would be still 36-5. 



If the sample had a specific gravity of 1-0335 or 33-5, it will 

 be found that the fat amounted to only 3-0 per cent., and the 

 sum would be 36-5. 



If the sample, however, contained only 3-8 per cent, of fat, 

 and had a specific gravity of 31-5, the sum would be only 35-3 ; 

 and it could then be concluded that the milk had been watered, 



OK o 



and that it contained only _--- X 100 = 97 per cent, of the 



OD'O 



original milk, or, in other words, 3 per cent, of water had been added. 



It has been found that this rule holds with remarkable accuracy 

 for any percentage of fat between and 10, and it is not very 

 far out with even higher percentages of fat. 



It must not be expected that the sum of the lactometer 

 degrees and fat will always add up to the same identical figure, 

 as there is a liability to error in both determinations ; with 

 care, however, the difference due to this cause should not 

 exceed 0-5. 



The value of the samples of series (3) lies in the fact that 

 rising of cream is most easily detected by their percentages of 

 fat being considerably different from that in series (]), as the 

 total effect due to this cause is usually marked. 



