142 ANIMALS AXD THEIR PRODUCTS. 



257. It will be recollected that soda is one of the 

 substances mentioned in a former section as contained 

 in milk. It is the office of the soda to hold the curd 

 in solution, a sort of imperfect solution, as before ex- 

 plained. Curd is not dissolved in pure water, but if 

 a little soda be added, the curd will to some extent 

 dissolve in it. 



258. It is so with milk ; it contains a little soda, in 

 a free state, that is, uncombined with anj other sub* 

 stance. The lactic acid, formed from sugar of milk 

 as before explained, combines with the soda, and neu- 

 tralizes its alkaline power, upon which the curd imme- 

 diately appears in the form of curdled milk. This, 

 if pressed, forms a kind of cheese. 



259. The milk then has in itself all that is absolutely 

 necessary to make cheese. This, however, would be a 

 slow, inconvenient process, and would not result in the 

 production of a good quality of cheese. The use of some 

 other acid than that naturally generated in the milk, 

 is therefore resorted to. It may be muriatic, or any 

 of the mineral acids ; or it may be a vegetable acid, 

 ns vinegar. The object of the acid is to neutralize 

 the soda, to strip it of its alkaline property, and thus 

 to withdraw it from its wonted office of holding the 

 curd in a kind of solution. 



260. I have said that almost any acid will answer 

 this purpose. A kind of animal acid, called rennet^ 

 taken from the stomachs of suckling calves, is more 

 commonly used. While the calf was living, the office 



