234 CHEMICAL INFLUENCES 



upon the accuracy with which we can prove the presence of a certain 

 bacterium. i 



Factors Influencing Disinfection. — ^The efficiency of a dis- 

 infectant depends upon several factors. Moisture is necessary — ■ 

 a dry poison has only a very slow action upon microorganisms. For 

 this reason, absolute alcohol has not nearly the same germicidal power 

 upon dry bacteria as diluted alcohol; the strongest poisonous effect 

 is obtained by a 50 to 70 per cent solution. The necessity of moisture 

 is further demonstrated in the sterilization with gases', as with formal- 

 dehyde. The effect of formaldehyde gas without the provision of a 

 very moist atmosphere is surprisingly weak. 



The temperature is also quite an important factor in the study 

 of disinfectants. Since poisoning is supposed to be a chemical effect, 

 it must be expected that the poisoning process like other chemical 

 processes will take place faster at a higher temperature. As a matter 

 of fact, the death rate through poisoning is usually doubled or trebled 

 by a temperature increase of 10°. Above the optimum temperature, 

 where the growth is not very vigorous, and when the disinfecting 

 power of the poison is increased considerably by the higher temperature, 

 a very small amount of poison will have a very strong germicidal effect. 

 The combination of high temperatures with a disinfectant has been 

 suggested as a means of sterilizing foods. This has been tried in 

 the case of milk with hydrogen perojdde at 50° to 60°. 



It makes a considerable difference whether the organisms which 

 are tested with a certain disinfectant are in a culture with their food 

 material, or suspended in yrater or salt solution without any food. It 

 is very probable that part of the disinfectant is acted upon by the food 

 products which are partly protein substances and are in many ways 

 similar to the protoplasm of the bacterial cells. It is especially diffi- 

 cult to poison bacteria in blood, pus, or similar material. The sensi- 

 bility of the microorganisms in pure water is remarkable. Very small 

 doses which would not be considered efficient under any other condition, 

 will destroy microorganisms in pure water. The concentration of 

 chloride of hme which is sufficient to sterihze drinking water, does 

 not a,t all suppress the development of bacteria in sewage. 



The influence of the number of cells is evident from the above ex- 

 planations of the mode of action, and from the curves of disinfection. 

 The concentration of the poison is of course of greatest importance. 



