CHAPTER IV. 

 THE ENVIRONMENT OF BACTERIA. 



Although bacteria are present almost everywhere upon 

 the surface of the globe it must not be supposed that they 

 are capable of growing wherever they may be found. The 

 air always contains a greater or less number of bacteria, 

 but these organisms are not multiplying. They are present 

 merely as extremely minute particles picked up, and wafted 

 about by currents. The conditions met with in the air are 

 far from being favorable to their development. On the 

 contrary the desiccation or drying, to which the floating 

 bacteria are subjected, is directly injurious to these 

 organisms. The sporeless bacteria are probably speedily 

 destroyed as a result of desiccation and of the action of 

 sun-light. 



Inasmuch as bacteria are specifically heavier than air 

 they tend to settle together with the coarser, floating matter. 

 The dust which therefore deposits on the floor, windows, 

 or upon the clothes, hair, or skin is rich in various bacteria, 

 yeasts and moulds. But even when thus deposited these 

 organisms will not grow and multiply. In order that they 

 shall grow it is necessary that they reach a suitable nutri- 

 ent medium and an essential condition of a nutrient soil is 

 that it shall contain moisture. 



Moisture, consequently, is absolutely necessary for the 

 development of bacteria. More than that, it is necessary to 

 the well being of every form of life, whether high or low. 

 This is seen in the fact that water enters largely into 

 the composition of living, growing protoplasm. The tis- 

 sues of higher plants and animals contain from 70 to 80 



