72 



ANALYSIS OF MILK. 



TABLE VII.— Specific Gravity and Volume of Milk. 



Mode of Averaging Specific Gravities. — It is not, ttere- 

 fore, correct in averaging millc analyses, where specific gravities 

 and percentages by weight are expressed, to obtain the average 

 specific gravity by adding the specific gravities together and 

 dividing by the total number, but specific gravities must be first 

 calculated to specific volumes, and these averaged, and the 

 average specific gravity deduced from the average specific 

 volume. 



Thus, to average the following analyses : — 



Specific gravity, 



1-022 

 1-036 



Total solids, . . £0-0 



10-0 



20 + 10 



The average total solids is — 



specific gravity is not 

 1 



= 15: 



but the average 

 1-022 + 1-036 , ^„„ , 1 

 ^ —^■^2^'^-*0:97&e^W3 



- 0-9719 = l-«289- 2 



The error is, however, small if the specific gravities do not 



differ greatly, and may very frequently be neglected. 



On the other hand, if, instead of averaging percentages of 



total solids by weight, we average the number of grammes per 



100 c.c, we obtain correct results by averaging the specific 



gravities. 



The following rules may be stated : — 



