194 AGRICULTURAL BACTERIOLOGY. 



brought on by climatic conditions, dependent chiefly upon the 

 temperature, and these same conditions are just those which 

 stimulate bacterial growth. It will thus happen that the same 

 sort of warm weather which produces the thunder storm also 

 hastens the growth of bacteria in milk if not kept artificially 

 cooled with ice. It will frequently happen, as a result, that 

 the milk will be ready to show signs of souring at the 

 same time that the thunder storm appears. The two phe- 

 nomena occur together, not because the one causes the other, 

 but because the same climatic conditions which produce the 

 storm hasten the growth of bacteria. A similar warm spell 

 will sour the milk just as quickly even though no thunder 

 storm appears. Whether this is the whole explanation ap- 

 pears a little doubtful, but it is clearly demonstrated that the 

 thunder and lightning have nothing to do directly with the 

 phenomenon. The souring of milk is always produced by 

 bacteria. 



PEPTONIZING AND RENNET-FORMING BACTERIA. 



The second type of normal bacteria in milk is one almost 

 universally found in milk, the number of whose species is 

 great, but whose significance in ordinary dairying is very slight. 

 The distinctive characteristic of these bacteria is the fact that 

 they secrete certain enzymes which have a decided action upon 

 milk. There are two such enzymes secreted by these bac- 

 teria, and some species of bacteria secrete both in considerable 

 quantity, while others appear to produce only one. 



The first of the enzymes is quite similar in its action to 

 trypsin secreted by the pancreas and has been called cascasc. 

 Its action is upon proteids which it converts into a soluble form 

 related to peptones. In the milk it acts upon the casein and 

 slowly renders it soluble, The milk under the action of this 

 enzyme becomes gradually more and more transparent, as the 

 casein is dissolved in the water, and finally it may become an 



