INFLUENCE OF PHYSICAL AGENTS. 155 



ference, in view of the fact that all known pathogenic bacteria and 

 their spores are quickly destroyed by the temperature of boiling 

 water ; and also that superheated steam is less effective than moist 

 steam. When confined steam in pJpes is ' ' superheated " it has about 

 the same germicidal power as hot dry air at the same temperature. 

 This is shown by the experiments of Esmarch, who found that an- 

 thrax spores were killed in streaming steam in four minutes, but 

 were not killed in the same time by superheated steam at a, tempera- 

 ture of 141 C. 



Desiccation. Cultures of bacteria kept in a moist condition re- 

 tain their vitality for a considerable time, which varies greatly with 

 different species. The writer has found that a culture of the typhoid 

 bacillus preserved in a hermetically sealed glass tube retained its 

 vitality for eighteen months, as did also Bacillus prodigiosus, Bacil- 

 lus cavicida, and some others. According to Kitasato, the cholera 

 spirillum may be preserved in a moist state for seven months ; other 

 bacteria die out in a month or two, but, as a rule, vitality is preserved 

 for several months at least. 



Spores in a desiccated condition preserve their vitality for a 

 great length of time. But desiccation is quickly fatal to some of the 

 pathogenic bacteria, and especially so to the cholera spirillum. Koch, 

 in his earlier experiments, found that his " comma bacillus" did not 

 grow after being dried upon a cover glass for three hours. Kitasato, 

 in experiments made since, found that a bouillon culture dried upon 

 a thin glass cover was incapable of development after three hours' 

 time, but that cultures in nutrient agar or gelatin survived for two 

 days, probably on account of the thicker layer formed and the longer 

 time required for complete desiccation. Pfuhl has found that the 

 typhoid bacillus dried upon a cover glass retains its vitality for 

 eight to ten weeks, and Lofner states that the diphtheria bacillus re- 

 sists desiccation for four or five months. Cadeac and Malet pro- 

 duced tuberculosis in guinea-pigs by injecting material from the 

 lung of a tuberculous cow which had been kept in the form of a dried 

 powder for nearly five months ; at a later date the virulence was 

 lost. 



Light. Downes and Blunt, in a communication made to the 

 Royal Society of London in 1877, first called attention to the fact that 

 light has an injurious effect upon bacteria, and that cultures may be 

 sterilized by exposure to direct sunlight. 



Tyndall, in experiments made in the clear sunlight of the Alps, 

 verified the fact that the development of bacteria was restrained in 

 cultures during their exposure, but failed to obtain evidence that 

 vitality was destroyed. 



In 1885 Duclaux took up the subject with pure cultures of various 



