258 MICRO-ORGANISMS AND DISEASE. [CHAP. 



at higher temperatures, such as 30 to 38 C. Heat above 

 50 or 60 C. arrests the growth of and even kills many 

 organisms, except the spores of bacilli, which, as we find on 

 a former page, survive even when exposed to the tempera- 

 ture of boiling water for several minutes. The presence of 

 carbolic acid, phenol, thymol, salicylic acid, perchloride of 

 mercury, &c., restrain even when in great dilution the growth 

 of micro-organisms. 



In any inquiry into the influence of one substance or 

 another on micro-organisms it is necessary to bear in mind 

 that the influence of certain conditions on the micro- 

 organisms may be a twofold one : (i) the condition may be 

 unfavourable to the growth of the organism in question, and 

 (2) the condition may be fatal to the life and existence of it. 

 The second condition involves, a fortiori, the first ; but the 

 reverse is not the case. Owing to the failure to distinguish 

 between these two propositions a great deal of confusion has 

 arisen on the subject. One hears constantly this or that 

 substance is an " antiseptic," meaning by this a substance un- 

 favourable to the growth of micro-organisms, or a substance 

 is a " germicide," implying by this that this substance kills the 

 organisms ; but when one comes to analyse the observations 

 that are said to establish this reputation for a particular sub- 

 stance, one finds that the substances in question have really 

 only a restraining effect on the growth of the organisms. 



By sowing any micro-organism into a nourishing medium, 

 to which has been added a certain substance (e.g. carbolic 

 acid to the amount of i per cent.), and exposing this 

 medium to the conditions of temperature, moisture, &c., 

 otherwise favourable to the growth of the organism, if we 

 find that after the lapse of a due period the growth is 

 retarded or altogether inhibited, the conclusion is drawn 

 that this substance (viz., the carbolic acid of i per cent.) is 



