778 BACTERIA 



is consequently only useful in preserving foods for a limited time, 

 and for the destruction of sporeless species of bacteria. 



(c) The vitality and work of bacteria are checked or destroyed 

 by various chemical substances ; those compounds which com- 

 pletely annihilate bacteria are termed disinfectants, while the 

 substances which merely retard the growth of these organisms 

 are known as antiseptics. No hard line of separation, however, 

 can be drawn between these two classes, for the effect of any 

 chemical compound depends upon the strength of its solution 

 and the time it is allowed to act. Moreover, what would be 

 sufficient to destroy or impede the growth of one species of 

 bacterium would not be effective on another species. 



The best disinfectants are mercuric chloride (o"2 per cent, 

 solution), sulphur dioxide and chlorine gases, phenol (lo per 

 cent, solurion), and formaldehyde : in weaker solutions the 

 above are antiseptics only, to which may be added salicyhc 

 acid, boric acid, milk of lime and alcohol. 



(d) Although those filters in use for domestic purposes are 

 generally worse than useless, it is possible to remove all bacteria 

 from liquids by filtrati' i through specially prepared apparatus. 



(e) Bright sunlight is destructive to bacteria and their spores. 



