86 THE INVISIBLE WORLD 



crobe of bread, but is much smaller. The air is 

 thickly populated with this species of microbes. 

 They as naturally like milk as pigs clover. As 

 soon as the milk comes from the cow, they are at- 

 tracted into it in great numbers. As soon as there, 

 too, they begin to grow and multiply with great 

 rapidity. One may become 3,000 in six hours. At 

 this rate, in the same time, a thousand become 3,- 

 000,000, and a million become 3,000,000,000. 



Coming into the milk so rapidly, and, at the 

 same time, multiplying so rapidly, the milk soon 

 becomes densely populated. By the time the milk- 

 man's milk gets to one's table, especially in warm 

 weather, a common glass of it will probably con- 

 tain not less than 50,000,000 of these beings. 



These microbes are the cause, and the only cause, 

 of all souring of milk. As soon as in the milk, 

 they subsist on the sugar of the milk, and, by their 

 life processes, convert this sugar into lactic acid; 

 and this acid is the essential element of all sour 

 milk. 



It is true that a number of different species of 

 microbes may, by converting its sugar into lactic 

 acid, sour milk. But ordinarily, and almost al- 

 ways, only one species is the active cause. Hence, 

 this species has been called " bacillus acidi aceti." 

 But the plain English name, microbe of milk, is 

 good enough for the present purpose. 



It is true that a number of other species oc- 

 casionally find their way from the air into the 

 milk, but only to do it harm. While they cause 



