16 SCIENCE AND PRACTICE OF DAIRYING. 



It is found that in milk standing for a time after milking, a coagu- 

 lation of the caseous matter takes place. The result of this is, that 

 the specific gravity of perfectly fresh milk, determined by means of 

 the hydrometer at 15 C. } will always be found to be higher, to 

 the extent of from *5 to one thousandth than in the same milk 

 when rapidly cooled or allowed to stand for some hours. For this 

 reason special precautions ought to be taken in testing the specific 

 gravity of milk with the hydrometer. The extent to which the 

 precipitation of the caseous matter takes place depends on the tem- 

 perature with a rising temperature it is increased, while with a 

 falling temperature it is diminished. For this reason, in the raising 

 of cream, equable low temperatures in the milk are not favourable, 

 because with low temperatures the fatty globules meet with increased 

 resistance in rising to the top. 



Among the more important early researches on the nature of the 

 albuminoids and caseous matter of milk may be mentioned those of 

 Scherer, Nasse, Schiitzenberger, Knop, and others. The theory first 

 advanced by Scherer in 1841, which was held for thirty years, that the 

 caseous matter is in the form of potassium albuminate, has now been com- 

 pletely controverted. The view which has been held on the subject of the 

 nitrogenous matter in milk, since 1875, is based on the reactions exhibited 

 by milk with certain reagents. 



If milk be precipitated, at the ordinary temperature, by dilute vinegar, 

 the larger portion of the nitrogenous matter is thrown down as a precipi- 

 tate. If the filtrate from this precipitate be heated, a second precipitate is 

 formed. The filtrate from this precipitate again gives a third precipitate 

 with alcohol; and by treating the filtrate from this last precipitate with 

 Millon's reagent, a fourth precipitate is obtained. It was consequently 

 believed that each one of these precipitates represented a separate 

 albuminoid, and these were distinguished as casein, albumin, albuminose 

 (Bouchardat and Quevenne), and lactoprotein (Millon and Commaille). 



But it may be pointed out, that the behaviour of the milk, as above 

 described, admits equally of the view which regards the nitrogenous 

 substance of the milk as consisting of one substance only. It merely 

 practically proves that the nitrogenous substance of the milk, at ordinary 

 temperatures, is only partially precipitated by vinegar, more completely 

 by vinegar at boiling temperature, and still more perfectly by alcohol, and 

 that it is completely precipitated by certain salts of the heavy metals. No 

 necessity exists, for inferring, on these grounds, the existence of four 

 separate albuminoid bodies, any more than for supposing, for example, 

 without further evidence, that there are four different kinds of lime, 



