BACTERIA. 87 



exceptional bacteria are known which appear to thrive in bright 

 light. This is true of the purple bacteria, which, like green 

 plants, assimilate carbon in presence of light. 



Bacteria are capable of living at considerable depths. 

 Russel found bacteria alive at a depth of 1,100 metres. They 

 are, therefore, capable of withstanding a pressure of over 

 100 atmospheres, and certain putrefactive bacteria have 

 survived a still higher pressure. 



With regard to the action of antiseptics on bacteria, the 

 rule has already been laid down, in the section on sterilisation, 

 that the higher the temperature the more easily they are 

 killed. But, with regard to the restrictive action of antiseptics, 

 it is usually weakest at the optimum temperature, and stronger 

 at both higher and lower temperatures. Very dilute solutions 

 of an antiseptic may encourage the growth of bacteria. 



Various species react differently to the same concentration 

 of a reagent, and the action depends to a great extent, with 

 any given species, upon the state of nourishment of the cells. 

 Spores are much more resistant than vegetative cells. It 

 has proved possible to propagate bacteria in the presence of 

 successively increased quantities of an antiseptic, but the charac- 

 ters so obtained prove not to be fixed, but disappear as soon 

 as the culture is prepared in a substratum free from poison. 



Bacteria and other micro-organisms when subjected to 

 mechanical vibration behave very variously. Horvath proved 

 that gentle vibration has no action on their growth, whereas 

 violent shaking hinders or entirely inhibits it. Meltzer arrived 

 at the conclusion, after prolonged experiments with liquid 

 cultures, that gentle vibration promotes the multiplication 

 of micro-organisms ; with a given degree of motion the rate 

 of germination of the species is at the maximum, whilst any 

 stronger vibration restricts it. The optimum and maximum 

 differ for each species. According to Appel, cultures of 

 bacteria on solid substrata behave the same whether they are 

 shaken or not. 



Attempts have been made ever since the discovery of 

 bacteria to define this large group of organisms, and to classify 

 the various species in one system, like other sections of the 

 vegetable kingdom. 



