46 



TNTRODUCTOEY — THE CONSTITUENTS OF MILK. 



the expansion of glass between 15-5° and 100° ; it has been 

 assumed that the volume of the glass instrument used to deter- 

 mine the density is the same at 100° as at 15-5°. The error has 

 no practical importance when the figures thus obtained from 

 different samples are compared, as they are all subject to the 

 same correction. 



From the average specific gravities given above the author 

 has calculated the true specific gravities and specific volumes ; 

 these are — 



The figures calculated are based on the assumption that the 



expansion is regular between 15° and 100°, and that it averages 



0-000863 for each degree Centigrade ; on this assumption the 



specific volume of liquid fat at 15° is higher, and the specific 



gravity is lower than that of the solid. 



15 '5° 

 The specific gravity of liquid fat at " ^ (calculated) is 0"922. 



It is interesting to note that the specific gravity of the liquid 

 fat obtained by the author (above) had a specific gravity of 

 ■0-92-2. The specific gravities of the liquid fats obtained by 

 Pizzi appear to have been taken at the temperature at which 

 the fat was separated, and on calculating to 15° have values from 

 0-921 to 0-925. 



On the whole the evidence available appears to show that the 

 specific gravity of solid fat is greater than that of liquid fat at 

 the same temperature. 



In connection with this, it may be mentioned that E. W. T. 

 .lones has shown that the specific gravity of other fats is greater 

 when partially solidfied at 37-8° than when liquid at this tem- 

 perature. 



The index of refraction of the fat of milk averages 1-4566 at 

 35°, and the limits observed have been 1-4550 to 1-4586. 



Stohmann has determined the heat of combustion of butter 

 fat as 9-231 calories per gramme, while Atwater found from 



