70 THE STORY OF GERM LIFE. 



warm and at a temperature which especially 

 stimulates bacterial growth. They multiply with 

 great rapidity, and in the course of a few hours 

 increase perhaps a thousandfold. The numbers 

 which may be found after twenty-four hours are 

 sometimes inconceivable ; market milk may con- 

 tain as many as five hundred millions per cubic 

 inch ; and while this is a decidedly extreme num- 

 ber, milk that is a day old will almost always 

 contain many millions in each cubic inch, the 

 number depending upon the age of the milk and 

 its temperature. During this growth the bacteria 

 have, of course, not been without their effect. 

 Recognising as we do that bacteria are agents for 

 chemical change, we are prepared to see the milk 

 undergoing some modifications during this rapid 

 multiplication of bacteria. The changes which 

 these bacteria produce in the milk and its prod- 

 ucts are numerous, and decidedly affect its value. 

 They are both advantageous and disadvantageous 

 to the dairyman. They are nuisances so far as 

 concerns the milk producer, but allies of the but- 

 ter and cheese maker. 



THE EFFECT OF BACTERIA ON MILK. 



The first and most universal change effected 

 in milk is its souring. So universal is this phe- 

 nomenon that it is generally regarded as an in- 

 evitable change which can not be avoided, and, as 

 already pointed out, has in the past been regarded 

 as a normal property of milk. To-day, however, 

 the phenomenon is well understood. It is due to 

 the action of certain of the milk bacteria upon 

 the milk sugar which converts it into lactic acid, 

 and this acid gives the sour taste and curdles 



