DENSITY. 3iS 



(37 "8° C), and this temperature is very near that found by Skal- 

 weit to give the largest difference. 



In America the temperature of 40° C. is used to a considerable 

 extent, and the author has taken a large number of densities at 

 39 '5° C. (owing to the use of a thermometer which read 0'5'" too 

 high). 



Estcourt proposed to use the temperature of boiling water 

 (which he found to raise the butter fat to 97-8° C. [208° F.]), as 

 being easily attained. Allen and others have warmly recom- 

 mended this temperature, and find no difficulty in bringing the 

 temperature up to 99° C. 



There is a certain amount of confusion as to the manner in 

 which densities are expressed. To ascertain the true density, 

 the weight of a certain volume of fat should be divided by the 

 weight of the same volume of water at the same temperature 

 and multiplied by the density of water at that temperature. 

 This is very rarely done, so that few published figures are true 

 densities. 



Muter gives the term " actual density " to the weight of a 

 certain volume of fat divided by the weight of the same volume 

 of water at the same temperature ; densities expressed thus 



37 "8° 

 are usually denoted by the symbol D ,,-^-^3 for density at 37 'X \ 



35° 37 



or D r^o for density at 35°, and the true density is often ex- 



4 T^37-8° ^35° 

 pressed as D „ or D - o"- 



It is usual when densities are taken at the temperature of 



boiling water to express them in a different way. The weight 



of a certain volume of fat is divided by the weight of water 



displaced by a piece of glass which occupies the same volume 



at the same temperature, when it is cooled down to 60° F. (15'5° 



C). This mode of expression may be denoted by the formula 



100° 

 D ^ o in glass. Though apparently cumbersome this method 



of expressing results has certain advantages, as the instrument 

 with which the densities are taken can be standardised at 60" F. 

 (15'5° C), and can then be used at any temperature without 

 requiring to be restandardised. It must be remembered that, 

 though the expansion of glass is very nearly constant, it is not 

 quite so, and over a range of 85° C. appreciable differences may 

 occur in the expansion of different instruments. If the glass 

 be not well annealed, internal strains are set up, and these may 

 be so accentuated at high temperatures as to cause distortion 

 and change of volume. It will be readUy seen that the method 

 of taking the apparent density in glass at the temperature of 



