340 MICRO-ORGANISMS IN WATER 



to 36 C. It should be mentioned that, in order to 

 obviate the formation of spores, the tubes were, during 

 the period over which the insolation extended, always 

 placed at night in a dark refrigerator. According to 

 Arloing, therefore, the spores are less capable of with- 

 standing the effect of light than the bacilli, an observa- 

 tion which is out of harmony with the commonly 

 accepted views concerning the greater powers of endur- 

 ance possessed by the former. In connection with the 

 individual characteristics of micro-organisms observed 

 in one and the same species which offer such a puzzling 

 phenomenon in the behaviour of bacteria, the following 

 observations of Arloing are of much interest, as being 

 an illustration of the manner in which the previous 

 history of a micro-organism affects its subsequent be- 

 haviour. 



It was observed that if a cultivation be insolated 

 for such a period of time which, whilst damaging it, 

 did not destroy it, the next generation derived from 

 this culture exhibited a tardiness in vegetation accord- 

 ing as the period of insolation to which the mother- 

 culture had been exposed was long or short. For 

 example, if the latter had been exposed to light for 

 from 4 to 8 hours, the development of the next genera- 

 tion was delayed for from 20 to 24 hours, whilst if the 

 original culture was exposed to from 15 to 20 hours' 

 insolation, its progeny were unable to exhibit any signs 

 of growth for from 36 to 40 hours. Nor was this all, 

 for an exposure of from 9 to 10 hours sufficed to kill a 

 generation the mother-culture of which had survived 

 25 hours' exposure to light. Arloing also ascertained 

 that these weakened cultures were less virulent when 

 inoculated into animals ; in fact, that after about 30 

 hours' insolation such anthrax cultures were converted 

 into a sort of vaccine, for guinea-pigs inoculated with 



