56 MICRO-ORGANISMS AND DISEASE. [CH. vi. 



have a deleterious effect. Small quantities impede their 

 growth, and sufficiently large quantities kill them altogether. 



Most bacteria are killed by heat below the temperature of 

 boiling water, many of them when exposed for several hours 

 to a temperature above 50 to 60 C. Exceptions are the 

 spores of bacilli, which in some instances (spores of hay 

 bacillus, Cohn) require exposure to the heat of boiling point 

 for as much as half an hour. By raising the boiling point 

 above 100, it does not require more than a few minutes to 

 kill them (Sanderson). 



Drying destroys most bacteria, except the spores of bacilli. 

 Freezing destroys likewise most bacteria, except the spores of 

 bacilli, which survive exposure to as low a temperature as 

 - 15 C., even when exposed for an hour or more. No spores 

 survive exposure to a temperature of 120 C. 



Amongst those substances which inhibit the growth of, or 

 altogether destroy the bacteria, are carbolic acid, salicylic 

 acid, thymol, &c. ; corrosive sublimate is one of the most 

 powerful, since even solutions as weak as i : 15,000 inhibit 

 the growth of some bacteria after exposure for thirty minutes. 



The best classification of bacteria is that given by Cohn, 1 

 and this we shall adopt : (i) spherobacteria or micrococci ; 

 (2) bacteria or microbacteria ; (3) bacilli or desmobacteria ; 

 (4) spirilla, (5) spirochsetse. There are also various kinds 

 which approach one or the other of these, e.g. ascococcus, 

 sarcina, leptothrix (Beggiotoa), cladothrix, streptothrix, &c. 

 (see below). 



I shall not attempt to give an exhaustive description of the 

 morphological characters of all micro-organisms, but shall 

 limit myself to those forms which are related in some way or 

 other to diseases. 



1 Beitr. z. Biol. d. PJl. Bd. i. 



