214 CHYMISTRY APPLIED TO AGRICULTURE. 



ARTICLE III. 



Of Caseous Matter. 



When milk has been skimmed, if it be afterwards heat- 

 ed to any degree short of ebullition, there form upon the 

 surface pellicles which gradually acquire some degree of 

 consistency, and which may be easily removed : by con- 

 tinuing the heat these may be renewed, till at length the 

 milk can furnish no more of them : in this state milk can 

 be boiled without occasioning any of that violent swelling 

 and rising which is so hard to check, and which causes 

 the boiling of this liquid to be so troublesome ; but then it 

 will contain neither butter nor caseous matter : the butter 

 has been separated in removing the cream, and the pelli- 

 cles are the caseous matter : what remains after these two 

 operations is only whey, holding in solution some known 

 salts. 



I have already remarked, that these pellicles form only in 

 contact with the air ; they do not appear when milk is boil- 

 ed in closely-corked bottles : the production of them may be 

 accelerated by the passage of a current of air over the sur- 

 face of the milk. 



The caseous matter may be separated from skimmed milk 

 by exposing it to a gentle heat, when it assumes the form of 

 a soft, quivering mass ; this is called curd : two or three 

 days' exposure to a heat of from 68° to 77° Fahrenheit is 

 sufficient to produce this effect. 



As. the caseous "matter adheres but slightly to the serum, 

 and to the salts which are contained in it, it can be sepa- 

 rated by means of a great variety of different bodies : it is 

 to the action of some one of these that recourse is had for 

 coagulating milk. 



Acids of all kinds coagulate skimmed milk very quickly ; 

 the change takes place more or less rapidly according to 

 the strength of the acid employed ; if a larger quantity be 

 used, the curd is injured by retaining the taste of the acid. 



The salts which contain an excess of acid, as the cream 

 of tartar, and the salts of sorrel, produce the same effects, 

 but the coagulation is not complete unless the milk is near 

 boiling when the salts are thrown into it. 



The rapidity with which the sulphates coagulate milk is 

 very remarkable : the action of these is most energetic upon 

 boiling milk. 



