vi] THE BACTERIA OF MILK 75 



naked eye when they occur in colonies in some fixed 

 medium. Many of them are less than 2 Q O th 

 of an inch in length, and few of them greater than 

 y-i-Q-th of an inch. Hundreds of millions of these 

 organisms could be spread over a square inch in 

 a single layer. A bacterium which causes lactic 

 fermentation is only about 2 5 \ & Q th of an inch 

 broad and 8 1 00 th of an inch long. No less, there- 

 fore, than twenty -five thousand of these bacteria 

 could be placed side by side without occupying more 

 than an inch, and nine hundred billions of them 

 would only weigh -^g-th of an ounce. 



Conditions influencing the Development of Micro- 

 Organisms. The conditions of life of micro-organisms 

 is a point which it is very important we should con- 

 sider, as our knowledge of this subject furnishes us 

 with the means of regulating their development. 



Bacteria may be divided into different classes, 

 according to their products. Thus a large class give 

 rise to pigments, another set up fermentation, and a 

 third class give rise to putrefactive decomposition. 



We can, however, divide them into two great 

 classes, according as they live on dead matter or 

 on living organisms. To the latter class belong the 

 pathogenic the disease-producing germs. Among 

 diseases that have been proved to be due to bacterial 

 life may be mentioned tuberculosis (consumption), 

 diphtheria, erysipelas, lockjaw, pneumonia, typhus, 



