76 INFLUENCE OF PHYSICAL AGENTS 



The spores (47) are always far more resistant than the vegetative cells, 

 and their resistance is proportional to their dryness. This must not be looked 

 upon as a peculiarity of bacterial spores, for all resting protoplasm is resis- 

 tant in consequence of the small amount of water it contains. Grains of 

 corn rendered as dry as possible in a desiccator can endure from 100 to 

 110 dry heat for hours without losing the power of germination. They are 

 in this respect not far inferior to anthrax spores, which succumb to 140 in 

 three hours. 



For many objects disinfection by dry heat is impracticable, as they 

 would be injured by the high temperature. The method can, however, be 

 advantageously used for disinfection of the glass ware used in bacteriological 

 work, but for surgical instruments and dressings boiling water or hot steam 

 is to be preferred. 



Spores are more quickly destroyed by moist heat, although if the 

 temperature of boiling water be not exceeded some hours are necessary to 

 destroy such resistant spores as those of B. subtilis and its allies. Anthrax 

 spores are generally killed by boiling water in from two to five minutes, but 

 one must always reckon on finding a few of much greater resistance that 

 need ten or twelve minutes. 



Moist seeds are much more quickly destroyed, being killed below 

 boiling point. The cause of the greater resistance of bacterial spores is not 

 known, but probably both the impermeability of the membrane and the 

 durability of the protoplasm play a part. If the impermeability of the 

 membrane be the cause, it is easy to conceive how the spores can remain 

 some time in water before the protoplasm can take up moisture. They 

 would even in boiling water be exposed at first to a dry heat. This view 

 gains in probability when we consider that the spores of R. subtilis germinate 

 very slowly, many hours passing before they swell and lose their lustre by 

 absorbing water. This stage is passed through more quickly if the spores be 

 boiled for five minutes. It seems as though the spore membrane were at 

 first very impervious to water. We know too that this is the case with the 

 resting spores of algae and fungi. The resting cells of other low organisms 

 such as amoebae, infusoria and flagellata, although not yet investigated, 

 are doubtlessly similar in this respect to bacterial spores. 



The sterilization of fruits and preserves by boiling in closed vessels 

 is familiar to all and has been in use since the last century. A discussion 

 of the various kinds of disinfectors, of the Koch steamer, and the autoclave 

 that by means of superheated steam kills the toughest spores in one minute 

 at 140 would be out of place here. They are used for the sterilization of 

 culture media. The uses to which these physical methods of disinfection are 

 put in public sanitation will be found in text-books of hygiene. 



All plants, those of our climates as well as those of the steppes and 

 deserts, have to protect themselves against dryness in times of drought. 



