64 THE STORY OF GERM LIFE. 



mentations which are ever interfering with our 

 industries, requiring special methods, or, indeed, 

 sometimes developing special industries to meet 

 them. As agents of decomposition, bacteria will 

 of course be a trouble whenever they get into 

 material which it is desired to preserve. Since 

 they are abundant everywhere, it is necessary to 

 count upon their attacking with certainty any 

 fermentable substance which is exposed to air 

 and water. Hence they are frequently the cause 

 of much trouble. In the fermentative industries 

 they occasionally cause an improper sort of fer- 

 mentation to occur unless care is taken to pre- 

 vent undesired species of bacteria from being 

 present. In vinegar making, improper species of 

 bacteria obtaining access to the solution give 

 rise to undesirable flavours, greatly injuring the 

 product. In tobacco curing it is very common 

 for the wrong species of bacteria to gain access 

 to the tobacco at some stage of the curing and 

 by their growth give rise to various troubles. 

 It is the ubiquitous presence of bacteria which 

 makes it impossible to preserve fruits, meats, or 

 vegetables for any length of time without special 

 methods. This fact in itself has caused the de- 

 velopment of one of our most important indus- 

 tries. Canning meats or fruits consists in noth- 

 ing more than bringing them into a condition in 

 which they will be preserved from attack of these 

 micro-organisms. The method is extremely sim- 

 ple in theory. It is nothing more than heating 

 the material to be preserved to a high tempera- 

 ture and then sealing it hermetically while it is 

 still hot. The heat kills all the bacteria which 

 may chance to be lodged in it, and the hermetical 

 sealing prevents other bacteria from obtaining 



