SPECIFIC GRAVITY. 79 



The specific gravity of separated milk may be corrected to 

 60 F. by using the column for 31 (between the black lines) ; 

 the reason for using this instead of the column proper to the 

 specific gravity, is that separated milk, being free from fat, 

 has a smaller expansion than milk. 



The author has devised a scale for correcting the specific 

 gravity of milk to 60 F. It is usually engraved on the " milk- 

 scale," and is used by adjusting the specific gravity found (on 

 the slide) to the arrow at 60 F. The corrected specific gravity 

 is found opposite the temperature at which the determination 

 was made. 



The corrected specific gravities obtained by the " milk-scale " 

 agree generally within 0-1 of those taken froiri the table. At very 

 low temperatures, however, there is sometimes a larger difference. 



S. H. Collins has devised a milk scale in which the temperature 

 correction for specific gravity is automatically made ; the points 

 denoting temperature and specific gravity observed are brought 

 together, and on the other side of the scale the percentage of 

 solids not fat corresponding to any percentage of fat is 

 read off. 



The Rise of Specific Gravity of Milk on Standing. 

 Milk drawn from the udder contains a large number of air bubbles, 

 and its specific gravity cannot be taken ; after the expiration of 

 an hour or so these have disappeared, and a specific gravity 

 determination is possible. It was first observed by Recknagel 

 that the specific gravity taken after the expiration of one hour 

 was lower than the specific gravity subsequently obtained. He 

 found the rise in specific gravity to be regular, more rapid at 

 low temperatures than high ones, and to amount on the average 

 to 0-001. He attributed the change to an alteration in the 

 volume of the casein. 



Vieth confirmed RecknagePs observation completely, and 

 found the average rise to be 0-0013 ; Bourcart also observed the 

 phenomenon. 



The author has studied Recknagel's phenomenon (as this 

 change in specific gravity has been called). In about 70 per 

 cent, of his experiments the rise in specific gravity has been 

 observed, varying from 0-0015 to 0-0003, and averaging 0-0006, 

 while in 30 per cent, of the observations no rise in specific gravity 

 was indicated. 



; The experiences of Babcock and Farrington agree with that of 

 the author. 



The author's experiments have confirmed the statement of 

 Recknagel, that the rise is more rapid when the temperature is 

 low than when high ; the same ultimate specific gravity is attained 

 whatever the temperature. 



