BACTERIA 143 



simple and can only be seen with the high powers of a micro- 

 scope. A good deal, however, can be made out regarding their 

 mode of life by simple experiments. Most bacteria can only 

 live when supplied with organic substances, in which they set 

 up decay (putrefaction). During this process gases are formed 

 which cause offensive smells ammonia, sulphuretted hydrogen 

 (from rotten eggs, cabbages, etc.), phosphoretted hydrogen (from 

 rotten fish, etc.) Other examples of bacterial action are the 

 souring of milk and the conversion of alcohol into acetic acid 

 (as in the souring of wine and cider). Bacteria are also concerned 

 in such processes as cheese-making and tobacco-* ' curing/' 



If you put a few peas into a tumbler of water and set it aside, 

 the water soon becomes cloudy, then a scum appears on the 

 surface, and after a time the water gets a nasty smell. The 

 spores, or " germs," of bacteria are present everywhere in the 

 air, ready to grow in any suitable material they may fall into. 

 New spores are formed in the scum on the water, and when the 

 water had all evaporated they would be blown about in the air, 

 in which they would remain suspended for a very long time, 

 since they are extremely small and light. The cloudy appear- 

 ance of the water is due to the presence of countless millions of 

 bacteria, for they multiply with great rapidity, and have minute 

 whip-like organs by means of which they swim about. We can 

 tell that they must have some power of movement, from the 

 fact that the cloudiness spreads through the whole of the water, 

 which would not be the case if the bacteria were motionless 

 and simply settled down in the water. 



As all know, some bacteria are of unpleasant importance 

 on account of the diseases they set up in mankind and in various 

 warm-blooded animals hardly a single plant-disease is due 

 to bacteria. But most bacteria are not merely harmless, but 

 of great use to mankind. The great work of bacteria is that 

 of breaking up the organic matter (remains of dead plants and 

 animals) in the soil. In this they are assisted to some extent 

 by moulds and other fungi, as well as such animals as earth- 

 worms and various insects, which, with the bacteria, make up 

 nature's army of scavengers. The ultimate products of decay 

 are largely used over again by plants as food. 



