386 EXPERIMENTS WITH PLANTS 



preservation of butter, fish, salt pork, corned beef, etc. 

 Inasmuch as salt does not kill the bacteria (but only 

 checks their growth), such flesh may contain disease- 

 producing bacteria, and be unsafe for eating. 



Sugar is an important preservative. In some dried 

 fruits, e. g., raisins, there is enough water to permit 

 the bacteria to grow were it not for the sugar which 

 checks them; the same is true of condensed milk. 



In addition to harmless preservatives such as salt, 

 sugar and vinegar, there are a number of injurious or 

 poisonous substances used, such as formalin, salicylic 

 acid and boracic acid. The public should insist that 

 pure food laws be made and enforced, to* prevent the 

 use of such preservatives. 



(c) Heat. — The important, practical application of 

 this is canning, in which the bacteria are destroyed by 

 heat, and the cans hermetically sealed. Tomatoes and 

 corn are difficult to can properly on account of the 

 presence of resistant spores which are not killed by the 

 heating, and which cause fermentation inside the can ; 

 the cans become swollen with gas so that the head 

 bulges out; such cans should always be rejected. 



{d) Cold. — The use of refrigerators by families, and 

 the construction of great cold-storage plants in cities, 

 illustrates the importance of this agent of preservation. 

 It should be remembered that cold does not kill many 

 kinds of bacteria, and that ice may be a source of 

 infection. For this reason it is always better to cool 



