96 THE INVISIBLE WORLD 



into the milk. The milk is well stirred. Then it 

 is kept quiet for the chemical change to take place. 



This change consists of two parts. The caseine 

 is distributed all through the milk in exceedingly 

 minute particles. As soon as the rennet is intro- 

 duced, it finds its way through all the milk to each 

 and every one of these particles. The moment the 

 particle of rennet comes in contact with the par- 

 ticle of caseine, it acts chemically upon that par- 

 ticle by splitting it into two parts. These parts 

 are called proteids. The one proteid is held only 

 in semi-solution, and is easily precipitated. The 

 other is held in closer solution, and is not pre- 

 cipitated at all. But it is so small that it is of 

 little or no account to the cheese-maker, while he 

 saves it in the whey. 



This completes the first part of the chemical 

 change. The work of the rennet is at an end. 

 But the caseine is not precipitated. The milk is 

 not curdled. Each and every particle of caseine 

 is still chained in semi-solution by the free alkali. 

 Happily the ash of the milk contains several salts. 

 One of these is the salts of calcium. The moment 

 this is set free, it acts chemically upon the free 

 alkali, neutralizing it. The chain that held in 

 semi-solution each and every particle of caseine is 

 thus broken. All the caseine is precipitated. The 

 milk is curdled. Both parts of the chemical 

 change are complete. The milk is changed into 

 curd and whey. With itself the caseine has taken 

 along the bulk of the butter, sugar and albumen. 



