INHIBITION OF GROWTH. 175 



figure 56 show the numbers of living spores of Bact. anthracis suspended 

 in mercuric chloride solution. The continuous solid curve gives the 

 result in a o . 2 1 per cent solution, the dotted curve indicates a o . 1 1 per 

 cent solution. The number decreases rapidly in the first few minutes, 

 but the rate of dying is not constant, and even after a comparatively 

 long time, a few spores remain alive. 



Similar curves are obtained in the destruction of microorganisms by 

 heat, by drying and by freezing. 



FACTORS INFLUENCING DISINFECTION. The efficiency of a disin- 

 fectant 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 40 to 50 per cent solution; bacteria suspended in water die the more 

 quickly the stronger the alcohol. The necessity of moisture is further 

 demonstrated in the sterilization with gases, as with formaldehyde. The 

 effect of formaldehyde gas without the provision of a very moist atmos- 

 phere is surprisingly weak. 



The temperature is also quite an important factor in the study of dis- 

 infectants. 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. This is actually the case. 

 It is possible to cool a disinfectant solution to the extent that it is only an 

 antiseptic. On the other hand, it is easily seen that above the optimum 

 temperature, where the growth is not very vigorous, and when the dis- 

 infecting power of the poison is increased considerably by the higher 

 temperature, a very small amount of poison will have a very strong germi- 

 cidal 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 peroxide 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 water or salt solution without any food. It is very pro- 

 bable 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 difficult to poison bacteria 

 in blood, pus, or similar material. The sensibility of microorganisms 

 in pure water is remarkable. Very small doses which would not be 



