GENERAL PRINCIPLES 433 



germicides ; other substances prevent their development and resulting 

 septic action, and are termed antiseptics. The word disinfectant is 

 used more or less indiscriminately to cover both these terms. ^ A 

 deodorant is, of course, a substance removing the odour of evil- 

 smelling putrefactive processes. These are the four common 

 designations of substances able to act injuriously on bacteria and 

 their products outside, or upon the surface of, the body. But a 

 moment's reflection will bring to mind two facts not to be forgotten. 

 In the first place, an antiseptic applied in very strong dose, or for an 

 extended period, may act as a germicide; and, vice versa, a 

 germicide in too weak solution to act as such may perform only 

 the function of an antiseptic. Moreover, the action of these dis- 

 infecting substances not only varies according to their own strength 

 and mode of application, but it varies also according to the specific 

 resistance of the protoplasm of the bacteria in question. Examples 

 of the latter are abundant ; for instance, between the bacillus of 

 typhoid fever and the spores of anthrax there is an enormous 

 difference in power of resistance. In the second place, there are 

 the physical conditions injurious to the development of bacteria. 

 At a low temperature bacteria do not multiply at the same rapidity 

 as at blood-heat. Within the limits of a moist perimeter the 

 air is, to all intents and purposes, germ-free. Direct sunlight has 

 a definitely germicidal effect in the course of time upon some of 

 the most virulent bacteria we know. In a certain sense these 

 three examples of physical conditions low temperature, moist 

 perimeter, direct sunlight may become first antiseptics and then 

 germicides. Yet for a limited period they have no injurious effect 

 upon bacteria. These would seem to be very simple points, and 

 calling for little comment, yet the pages of medical and sanitary 

 journals reveal not a few keen controversies upon the injurious 

 action of certain substances upon certain bacteria, owing to the 

 discrepancies of necessity arising between results of different 

 skilled observers who have been carrying out different experiments 

 with different solutions of the same substance upon different proto- 

 plasms of the same species of bacteria. We feel no doubt that in 

 these pioneering researches much labour has been to some extent 

 misspent, owing to the neglect of a common denominator. Only a 

 more accurate knowledge of bacteria or a recognised standard for 

 disinfecting experiments can ever supply such common denominator. 

 Species of bacteria for comparative-observation-experiments into the 

 action of chemical or physical agents must be not only the same 

 species, but cultured under the same conditions, and treated by the 

 agent in the same manner, otherwise the results cannot be compared 

 upon a common basis, or with any hope of arriving at comparable 

 conclusions. 



2 E 



