430 LIQUID PARTS OF ANIMALS. 



In a third column is inserted the weight of water contained in 

 100 parts of the respective milks. 



Cream consists of a peculiar oily matter mixed with curd and 

 whey, and the substances held in solution in the whey. When 

 agitated for some time it separates into two portions, namely, a 

 solid yellow substance called butter, and a liquid portion contain- 

 ing the greatest part of the curd and whey. This liquid is cal- 

 led butter-milk. The process itself is called churning. 



The formation of butter goes on equally well whether the ac- 

 cess of air be admitted or precluded. Macaire Princep has as- 

 certained by experiment, that no oxygen is absorbed from the at- 

 mosphere during the process of churning. This indeed has long 

 before been shown by Young* and by Thenard.f 



In some cases it is said that there is an extrication of gas dur- 

 ing the churning of butter, and it has been inferred that this gas 

 is carbonic acid. But the fact has not been established in a sa- 

 tisfactory manner. Dr Young affirms, that during churning 

 there is an increase of temperature amounting to 4 degrees. 

 Cream, according to the analysis of Berzelius, consists of, 



Butter, ' 4-5 



Curd, i; - 3-5 



Whey, 92-0 



100-0 



but it varies so much in the proportion of its constituents that 

 such analyses are of very little value. 



The appearance and characters of butter are so universally 

 known that it is needless to describe it. In its usual state it con- 

 tains about Jth of its weight of substances contained in butter- 

 milk. To separate the butter from these substances it is to be 

 put into a cylindrical glass, and raised to a temperature which 

 must not be higher than 140. The butter melts and swims up- 

 on the surface under the form of an oil, while the butter-milk is 

 collected in the lower part of the vessel. When the butter oil 

 has become clear it is to be poured into another vessel contain- 

 ing water heated to 1 04, with which it is to be well-agitated, in 

 order to separate every thing from it that is soluble in that li- 

 quid. When the mixture is left at rest the butter -oil collects on 

 the surface, and when the water cools concretes into solid butter. 



* Young de Lacte, p. 15. f Nicholson's Journal, xii. 218. 



