144 AGRICULTURE. 



substance separates and leaves a bluish water behind. The 

 cream or butter, then, is an oil or fat which is mixed through 

 the milk, and from the fact that it comes to the top we conclude 

 that it is lighter than the skim-milk. It is not dissolved in the 

 water of the milk as sugar is dissolved in water, but is simply 

 mixed with it or distributed through it in very fine particles ; 

 in fact, we can put it back into the skim-milk if we pour the 

 two together from one vessel into another before the milk 



sours. It is in the form of 

 what is called an "emulsion." 

 When fresh milk is run through 

 a cream separator, the heavy 

 skim-milk is thrown away from 

 the lighter fat or cream. This 

 cou ld not be done if the fat were 

 dissolved in it. Milk, then, 

 contains water and fat or oil 

 butter-fat, as it is called. Now 



Fig. 77. Milk, showing the fat globules take SOme skim-milk and Slight- 

 floating in it. . A i r 



ly warm it. A thin scum forms 



upon it. This scum is composed principally of albumen, a 

 nitrogen compound similar to the white of egg, which becomes 

 white and nsoluble by heating or cooking. It forms but a 

 small portion of the milk. If, however, we put a few drops of 

 rennet or vinegar into the skim-milk, a curdling at once takes 

 place, and a considerable quantity of material is thrown out of 

 solution and floats about as a curdy or cheesy mass ; this 

 is the casein of the milk, also a nitrogen compound. Then 

 we have at least two nitrogen compounds in milk the 

 albumen, which is curdled by heat, and the casein, which is 

 curdled by acids. The latter is in much larger quantity than 

 the former, and both are in solution in the water of the milk. 

 We can readily prove that these two contain nitrogen, and 

 differ therein from the fat. Take some pure butter and burn 



