1900.] on the Effect of Physical Agents on Bacterial Life. 455 



75° C. These remarks apply to the bacteria during their multiply- 

 ing and vegetating phase of life. In their resting or spore stage 

 the organisms are much more resistant to heat. Thus the anthrax 

 organism in its bacillary phase is killed in one minute at 70° C. ; in 

 its spore stage it resists this temperature for hours, and is only killed 

 after some minutes by boiling. In the soil there are spores of 

 bacteria which require boiling for sixteen hours to ensure their death. 

 These are important points to be remembered in sterilisation or 

 disinfection experiments, viz. whether an organism does or not 

 produce these resistant spores. Most non-sporing forms are killed at 

 60° C. in a few minutes, but in an air-dry condition a longer time is 

 necessary. Dry heat requires a longer time to act than moist heat : 

 it requires 140° C. for three hours to kill anthrax spores. Dry heat 

 cannot therefore be used for ordinary disinfection on account of its 

 destructive action. Moist heat in the form of steam is the most 

 effectual disinfectant, killing anthrax spores at boiling point in a few 

 minutes, whilst a still quicker action is obtained if saturated steam 

 under pressure be used. No spore, however resistant, remains alive 

 after one minute's exposure to steam at 140° C. The varying thermal 

 death-point of organisms and the problems of sterilisation cannot be 

 better illustrated than in the case of milk, which is an admirable soil 

 for the growth of a large number of bacteria. The most obvious 

 example of this is the souring and curdling of milk that occurs after 

 it has been standing for some time. This change is mainly due to 

 the lactic acid bacteria, which ferment the milk sugar with the pro- 

 duction of acidity. 



Another class of bacteria may curdle the milk without souring 

 it in virtue of a rennet-like ferment, whilst a third class precipitate 

 and dissolve the casein of the milk, along with the development of 

 butyric acid. The process whereby milk is submitted to a heat of 

 65° to 70° C. for twenty minutes is known as pasteurisation, and the 

 milk so treated is familiar to us all as pasteurised milk. Whilst 

 the pasteurising process weeds out the lactic acid bacteria from the 

 milk, a temperature of 100° C. for one hour is necessary to destroy 

 the butyric acid organisms: and even when this has been accom- 

 plished there still remain in the milk the spores of organisms which 

 are only killed after a temperature of 100° C. for three to six hours. 

 It will therefore be seen that pasteurisation produces a partial, not 

 a complete sterilisation of the milk as regards its usual bacterial 

 inhabitants. The sterilisation to be absolute would require six 

 hours at boiling point. But for all ordinary practical purposes 

 pasteurisation is an adequate procedure. All practical hygienic re- 

 quirements are likewise adequately met by pasteurisation, if it is 

 properly carried out, and the milk is subsequently cooled. Milk 

 may carry the infection of diphtheria, cholera, typhoid and scarlet 

 fevers as well as the tubercle bacillus from a diseased animal to the 

 human subject. For the purpose of rendering the milk innocuous, 

 freezing and the addition of preservatives are inadequate methods 



