78 



PHYSICAL DETERMINATIONS. 



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

 differ greatly, and may be very frequently 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 : 



(1) If equal volumes of different milks be mixed, the specific 

 gravity of the mixture will be the mean of the specific gravities 

 of the milks. 



(2) If equal weights of different milks be mixed, the specific 

 volume of the mixture will be the mean of the specific volumes 

 of the milks. 



The Alteration of Specific Gravity by Change of 

 Temperature. Milk, like all other substances, alters in specific 

 gravity by change of temperature. 



Though it contains a large amount of water, it does not share 

 the anomaly which this substance possesses of attaining its maxi- 

 mum specific gravity at 4 C. (39 F.). It decreases in specific 

 gravity when heated from its freezing point 0-55 C. (31 F.). 



The following figures (Table XIV.) show the average apparent 

 expansion of milk in glass : 



TABLE XIV. EXPANSION OF MILK. 



The expansion is greater with rich milk than with poor milk ; 

 the above figures referring to milk having a specific gravity of 

 1-032 and containing 3-8 per cent. fat. 



Table XV. affords a means of correcting to 60 F. the specific 

 gravity of milk when taken by a lactometer at any temperature 

 from 40 F. to 80 F. The table gives specific gravities from 1 -025 

 (25 degrees) to 1 -036 (36 degrees) and is applicable to whole milk 

 only. 



The table is used by looking up the degrees of specific gravity 

 found (or the nearest whole degree) in a horizontal line, and the 

 temperature in a vertical line ; the figure at the intersection 

 of the two lines is the correction to be added if above 60 and 

 subtracted if below to reduce the specific gravity to 60 F. 



