YORK SOCIETY. 67 



allow it to stand at rest for from forty to sixty hours, when near- 

 ly all the oil (or butter) will rise to the surface, owing to its being- 

 lighter than the other substances. This proves that the oil or 

 butter in the form of cream is not chemically united with the 

 water, but merely floats in it. Tlie butter ordinarily amounts 

 to about one part in fifteen to twenty parts of milk, though 

 instances are given where seven to ten pounds of milk have 

 yielded one pound of butter. 



Remove the cream and keep skimmed milk from souring and 

 the curd (casein) will not separate from the water. But add 

 any sour substance, such as vinegar or any of the acids, and it 

 will at once curdle — that is, the curd will separate into a mass 

 by itself. Take out this curd and put it into pure water, and 

 it will not be dissolved again; but add some soda or sala3ratus 

 to the water, and it will then dissolve the curd. If we examine 

 new milk, or uncurdled skimmed milk, chemically, we shall find 

 that it contains a little soda. These and other experiments 

 show that casein or curd is not dissolved in pure water, but 

 that it is held in solution in milk by means of the free soda 

 present. If milk is allowed to stand exposed to air for a time,, 

 it sours of itself — that is, some of its own elements change to 

 an acid, and this acid neutralizes or destroys the free soda 

 existing naturally in the milk, and the water not being able to 

 dissolve the casein or curd without the soda, the casein sepa- 

 rates into a "curdled" mass, just as camphor in solution sepa- 

 rates into a flocky mass when water is poured in to withdraw 

 the alcohol. By adding soda to milk faster than it is used up- 

 by the acid formed in souring, we can keep milk from curdling 

 weeks or months. About twenty-five pounds of milk will pro- 

 duce one pound of dry curd. Cheese is curd, containing some 

 water and more or less oil or butter. 



After the oil and curd (casein) is removed, we have left what 

 is usually called whey, which is chiefly water containing sugar 

 and the small quantity of salts above described. Carefully 

 evaporate the whey to dryness, and we shall have left a quan- 

 tity of sugar not unlike common white sugar, though a little 

 less sweet to the taste. There will be a little more than one 

 pound of this sugar to twenty-five pounds of milk or whey. 

 Mingled with the sugar will be a little more than an ounce of 



