308 MILK AND MILK PRODUCTS. XIV. 



liquid in the animal. Its specific gravity varies, but is usually 

 930 at 15, compared with water at the same temperature, 



07.00 RQ-^ 



at ~ (liquid) = -9118, at ~ = '9113. 



Solid fat is heavier, volume for volume, than the liquid form 

 -at the same temperature (Bichmond) ; so that evidently con- 

 traction occurs at the moment of solidification. By very slow 

 cooling of melted butter fat a partial separation of the various 

 glyceryl salts occurs, the portion solidifying first being charac- 

 terised by containing less volatile acids and less oleic acid or 

 other unsaturated acids than the portion remaining liquid. 



The index of refraction of milk fat varies from 1-4550 to 

 1-4586 at 35; the heat of combustion of 1 gramme is 9231-3 

 calories/ 1 ' It is soluble in hydrocarbons, in ether, carbon 

 disulphide, acetone, nitrobenzene, and in warm arnyl alcohol. 



The composition of butter fat is liable to considerable varia- 

 tion, being affected by the food, period of lactation, and other 

 conditions affecting the cows. It has been observed that large 

 quantities of cotton cake have a marked effect upon the butter 

 and cause it to become harder and whiter, and to give the re- 

 actions for cotton-seed oil. This effect has been noticed within 

 24 hours after feeding with cotton cake commenced. ! Sesame- 

 oil cake, almond cake, and cocoanut-oil cake used as food for 

 cows alter the iodine value and percentage of volatile fatty 

 acids of the butter fat.J Eeference to the change in composi- 

 tion of butter fat with advancing lactation has already been 

 made. 



The fat exists in the milk as minute globules of diameters 

 varying from -0016 to -010mm. The number of globules in 

 milk is astonishingly great, being estimated by different 

 observers at from 1-52 to 11-4 millions in the cubic millimetre. 

 The globules vary greatly in size in any particular sample, but 

 certain breeds of cows are remarkable for the preponderance of 

 large-sized or of small-sized globules. It has been suggested 

 that the fat globules are surrounded by an albuminous mem- 

 brane, but this theory does not receive much support at 

 present, and the generally accepted view is that the fat is in 



* Stohman & Langbein, Jour. Chem. Soc. 1891 abst. 11. 



t Thorpe, J.C.S. 1900, abst. ii. 237. 

 J Baumertand Falke, Zeitschrift Untersuch. d. Nahrungs- & Oenussm. 1898, 065. 



