14 SCIENCE AND PRACTICE OF DAIRYING. 



If a candle light be looked at through a thin layer of milk, the flame 

 usually appears yellow, but occasionally it appears of a reddish colour. 

 The thickness of the milk layer with which this takes place is dependent 

 upon the percentage of fat the milk contains, but is not directly propor- 

 tional to its amount, as it is also dependent upon the size of the fatty 

 globules present. The same quantity of fat retards more light when it is 

 in the form of very small globules, than when it is in the form of larger 

 globules. It is for this reason that the determination of fat by the so-called 

 optical method is so very unreliable. 



According to Jorgensen, the refractive index of milk serum lies between 

 1-3470 and 1-3515, and that of curd, coagulated by rennet, between 1-3433 

 and 1-3465. 



It may be taken for granted, that the suspended matters of milk 

 the fat, the nitrogenous substances, and the phosphate of lime have the 

 same effect upon the chemical balance and on the hydrometer as if they 

 were in solution, although this does not necessarily follow as a self-evident 

 fact. Mach has shown that very finely divided bodies suspended in liquids 

 only exert their weight on the balance and areometer when they are either 

 at rest, or are moving with a regular speed. That these conditions are 

 fulfilled by the substances in suspension in milk is proved by the fact that 

 tests of the specific gravity of milk conducted in a most careful way, both 

 with the balance and with the hydrometer, give constant and perfectly 

 concurrent results. 



It is noteworthy that milk, rich in fat, despite this richness in a 

 constituent of low specific gravity, does not generally exhibit a low specific 

 gravity, nor milk poor in fat, a high specific gravity. This is owing to the 

 fact that milk rich in fat is also rich in the other solid constituents, and 

 milk poor in fat is also poor in the other constituents. The specific gravity 

 of milk is always exactly proportional to the percentage of the non-fatty 

 solids. W. Thorner has investigated the resistance which milk offers to 

 the electric current, and has found that the resistance of pure milk is not 

 an absolutely constant quantity. It is more or less increased by the 

 addition of water, diminishes with increasing acidity of the milk, and is 

 independent of the amount of fat it contains. 



5. The Nitrogenous Matter in Milk. This forms from 2'5 per 

 cent to 4*2 per cent on an average 3'5 per cent of the contents of 

 milk, and consists of substances of the nature of protein the 

 so-called albuminoids. Duclaux's theory, that there is only one 

 albuminoid in milk, is not consistent with the properties exhibited 

 by it. It is highly probable that milk contains three albuminoids 

 casein, lactalbumin, and globulin the casein being very much in 



