274 THE PHYSICAL EXAMINATION OF BUTTER FAT. 



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 

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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 readily seen that the method 

 of taking the apparent density in glass at the temperature of 

 boiling water is liable to greater experimental error than deter- 

 minations at lower temperatures, and, as the experiments of 

 Skalweit have shown, that the effect of experimental error is 

 magnified at 100 C., owing to there being a smaller difference 

 between the densities of butter and margarine at this tempera- 

 ture than at lower ones. It is desirable not to adopt this method 

 where accuracy is, as it always should be, a desideratum. 



On the whole, it seems desirable to adopt 100 F. (37-8 C.) as 

 the standard temperature at which determinations should be 

 made, because it is sufficiently near Skalweit's minimum to give 

 a large difference between butter and margarine, and because a 

 large number of experiments on genuine butters have already 

 been made at this temperature. 



Determination. The density of butter is best determined by 

 the pycnometer. This is filled with distilled water, and the 

 weight of the water which it holds at 37-8 determined. After 

 drying, by placing in the water oven and drawing a current of 

 air through it, it is filled with the fat and placed in water at 

 37 -8 C. till the volume is constant ; the temperature must be 

 accurate to 0-1 C. if the result is required to be exact to the 

 fourth place of decimals. The. weight of fat divided by the 



37'8 

 weight of water will give the density at . 



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The Westphal balance may be employed, the apparent density 

 of water at 37-8 must be determined, and the density of fat 

 indicated by the instrument divided by this to obtain the density 



37-8 

 at 37-8' 



