MILK-FAT. 23 



ally at not less than 41 C. to 42 C. In the case of most other fats the 

 melting-point is higher. The majority of the insoluble fatty acids which 

 make up milk-fat (palmitic, stearic, and oleic acids) melt at temperatures 

 between 38 C. and 39 C., or, according to Adolf Mayer's researches, 

 between 41 C. and 44 C., and become solid between 35 C. and 38 C. 

 The solidifying point of milk-fat lies, as a rule, between 19 C. and 24 C. 

 It may, however, vary between 27 C. and 12 C. At the moment of 

 solidification only a slight rise in temperature takes place, which proves 

 that the latent heat of milk-fat is very slight. The consistence and colour 

 of milk-fat depend on the influence of feeding, and vary according to the 

 lapse of time after the lactation period. They appear also to be dependent 

 on the age and individuality of the animal. The melting point of milk-fat 

 is said to be lowered by feeding with easily digestible carbohydrates, and 

 raised by feeding with fodders poor in sugar, such as straw and oil- 

 cakes. 



The specific gravity of milk-fat in air at 15 C. (distilled water taken at 

 the same tempera ture = 1) is, on an average, '930717; and in vacuum 

 (water taken at 4 C. as 1) it is, on an average, -93002. At the boiling 

 point of water, and at a barometric pressure of 760 mm., reduced to C., 

 it varies between '8650 and '8685. Most of the other fats, at the boiling 

 point of water, possess a specific gravity of less than -8610. According to 

 M. Schrodt, the refraction exponent of milk-fat is only subject to small 

 variations, and is neither affected by the feeding of the cow, nor by the 

 lactation period, and is, at 22 C., on an average, 1'4590. With the 

 diminution of the percentage of the fatty acids it appears to increase. If 

 pure milk-fat be preserved from the action of air for some time, it becomes 

 rancid, that is, decomposition sets in, and small quantities of volatile fatty 

 acids, and particularly butyric acid, are set free. Free exposure to air and 

 sunshine hastens the decomposition. Under such conditions volatile fatty 

 acids, directly derived from the glycerides of butter-fat, are also set free, 

 and other fatty acids, of which formic acid is probably one, are formed, 

 oxygen being absorbed from the air. Milk-fat, in this process of decom- 

 position, possesses not merely a rancid and strongly tallowish smell and 

 taste, but also assumes a white colour. Certain moulds, when the milk 

 becomes infected with them, produce decomposition, which is accompanied 

 by a gradual diminution of the volatile fatty acids of the milk-fat. In 

 this process butyrin shows itself to be less easily decomposed than capronin, 

 and the latter less easily decomposed than the neutral fats of the essential 

 fatty acids. 



Although the hypothesis that the larger and the smaller of the fatty 

 globules of milk contain different kinds of fat, has not, so far, been con- 

 clusively proved, it has become more and more probable. The fat of the 



