388 MICRO-OKGANISMS AND DISEASE. [CHAP. 



&c., prevent 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 : (1) the condition may be unfavourable 

 to the growth of the organism in question, and (2) the con- 

 dition 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 011 the 

 subject. One hears constantly this or that substance is an " an- 

 tiseptic," meaning by this a substance inimical 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 substance, one finds that the 

 .substances in question are really only detrimental to 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 1 per cent.), and exposing this medium to 

 the conditions of temperature, moisture, &c., otherwise favour- 

 able 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 1 per cent.) is an antiseptic. There is nothing 

 more fallacious than this method of reasoning ; a great many 

 micro-organisms can be exposed to a 1 per cent, solution of 

 carbolic acid for hours without in the least being affected, for 

 on being then transferred to a suitable nourishing medium 

 they grow and thrive well. Similarly by placing the spores 

 of bacillus anthracis in a proteid medium containing per- 

 chloride of mercury of the strength of 1 in 300,000, it is 

 found (as Koch has shown) that the spores are absolutely 

 incapable of germinating. But if from this the conclusion is 

 drawn, that perchloride of mercury of the strength of 1 in 

 300,000 is a germicide, I should most strongly dissent, for 

 perchloride of mercury even' of the strength of 1 per cent, is 

 not a germicide any more than vinegar ; for on placing the 

 spores of bacillus anthracis in a proteid medium, to which so 

 much vinegar or any other acid has been added as makes 

 it decidedly acid, it will be found that the spores do not 

 germinate. 



