154 INFLUENCE OF PHYSICAL AGENTS. 



In considering the influence of high temperatures we must take 

 account of the very great difference in the resisting power of the 

 vegetative cells and the reproductive elements known as spores, also 

 of the fact as to whether dry or moist heat is used and the time of 

 exposure. 



Dry Heat. When microorganisms in a desiccated condition are 

 exposed to the action of heated dry air, the temperature required for 

 their destruction is much above that required when they are in a 

 moist condition or when they are exposed to the action of hot water 

 or steam. This was thoroughly demonstrated by the experiments of 

 Koch and Wolff hugel (1881). A large number of pathogenic and 

 non-pathogenic species were tested, with the following general result : 

 A temperature of 78 to 123 C. maintained for an hour and a half 

 (over 100 for an hour) failed to kill various non-pathogenic bacteria, 

 but was fatal to the bacillus of mouse septicaemia and that of rabbit 

 septicaemia. To insure the destruction of all the species tested, in 

 the absence of spores, a temperature of 120 to 128 C., maintained 

 for an hour and a half, was required. 



The spores of Bacillus anthracis and of Bacillus subtilis resisted 

 this temperature and required to insure their destruction a tempera- 

 ture of 140 C. maintained for three hours. This temperature was 

 found to injure most objects requiring disinfection, such as clothing 

 and bedding. But the lower temperature which destroys micro- 

 organisms in the absence of spores (120 C. =. 248 F.) can be used 

 for disinfecting articles soiled with the discharges of patients with 

 cholera, typhoid fever, or diphtheria, as the specific germs of these 

 diseases do not form spores. It is probable also that it may be safely 

 used to disinfect the clothing of small-pox patients, for we have ex- 

 perimental evidence that a lower temperature destroys the virulence 

 of vaccine virus (90-95 C. Baxter). 



In practical disinfection by means of dry heat it will be necessary 

 to remember that it has but little penetrating power. In the experi- 

 ments of Koch and Wolffhiigel it was found that registering ther- 

 mometers placed in the interior of folded blankets and packages of 

 various kinds did not show a temperature capable of killing bacteria 

 after three hours' exposure in a hot-air oven at 133 C. and above. 



Moist Heat. The thermal death-point of bacteria, in the ab- 

 sence of spores, is comparatively low when they are exposed to moist 

 heat. The results of the writer's experiments are given below : 



" In my temperature experiments I have taken great pains to insure the 

 exposure of the test organisms to a uniform temperature, and have adopted 

 tt-n minutes as the standard time of exposure. The method employed 

 throughout has been as follows: From glass tubing having a diameter of 

 about three-sixteenths of an inch I draw out in the flame of a Bunsen burner 

 a number of capillary tubes, with an expanded extremity which serves as 



