86 OUTLINES OF BACTERIOLOGY 



the room. The bacteria are caught and are in much the same position 

 as flies in a bowl of milk. 



Disinfectants. We must distinguish between an antiseptic and a 

 germicide, the latter being used to designate an agent that kills bacteria, 

 whereas the former applies to one which hinders their development. 

 It is like the difference between merely stunning a man and killing him 

 outright, and naturally, the latter is the more effective of the two. 

 But, just as a stunned man may recover his senses, so, it must be 

 borne in mind, may the bacteria, treated with antiseptics, recover their 

 strength and multiply, should they find themselves in a suitable 

 medium. 



It will be well, first of all, to consider the conditions that are 

 inimical to bacterial life. These are : 



1 . Very high and very low temperatures. 



2. Light. 



3. Chemical substances that act as poisons. 



4. Very long continued dearth of water. 



Two of these, however, cold and drought, may be dismissed at once 

 as practical disinfectants, on account of the very long time which would 

 be required for dearth of water or want of heat to result in destruction 

 of bacteria. As, however, no multiplication of bacteria could take place 

 under these conditions, they may be regarded as slight antiseptics. 

 Thus, it has been shown that the tubercle bacillus can stand the 

 temperature of liquid air (-193°C.) for 42 days without losing its 

 vitality ; and the spores of bacteria have been found to possess the 

 power of germination after being in a dry condition for several years. 



Heat, however, is a practical disinfectant. When the temperature 

 of protoplasm reaches 55° C, in the case of all except the thermophilous 

 or heat-loving bacteria, death ensues. Also, in the case of even the 

 thermophilous kinds, the death-temperature is not very much higher ; 

 hence, we have only to heat the substance until conduction has effected 

 a sufficient rise of temperature in the protoplasm. As the temperature 

 of boiling water at normal atmospheric pressure is 100° C, sterilising 

 by its means is very effective in the case of non-sporogenous bacteria, 

 to which, fortunately, almost all the pathogenic bacteria belong. It is 

 far more difficult, however, to kill bacteria when they are in the sporo- 

 genous condition, for the outer of the two coats of the spore is an 

 extremely bad conductor of heat, so that much more prolonged heat is 

 required for efficient sterilisation. 



The following table shows the difference between the resisting power 

 of bacteria when in the sporogenous and when in the non-sporogenous 



