352 MICKO-ORGANISMS IN WATER 



Laurent ^ on a bacillus originally found in water {Bacille 

 rouge de Kiel, see Appendix, p. 442) must be here referred 

 to. This bacillus was inoculated on to slices of potato 

 which were then exposed during 1, 3, and 5 hours re- 

 spectively to the direct rays of a July sun, after which 

 they were placed in the incubator at 33° C. On the 

 potato exposed to the sun for one hour, white colonies 

 mixed with a small number of pink centres made 

 their appearance. On the potato exposed for three 

 hours, colourless colonies, with the exception of a few 

 which were pale pink, developed ; whilst it was found 

 that five hours' insolation had entirely destroyed the 

 bacilli on the third slice. The white colonies from 

 both the slices were inoculated on to slices of pota- 

 toes and kept in the incubator at 33° C. Nearly all 

 the colonies resulting from the white colonies of the 

 slice insolated for one hour were pink, whereas, with 

 but one or two exceptions, the colonies inoculated from 

 the slice exposed for three hours were colourless, and 

 after being transplanted for the third time on to 

 potatoes no trace of colour in the growth remained. 

 Laurent says that the light had so far modified the 

 physiological character of the bacillus that a colourless 

 race, perfectly uniform in its behaviour in this respect, 

 had been obtained. As many as 32 successive colour- 

 less generations of this bacillus were obtained on slices 

 of potato kept in the incubator at from 25°-35° C, but 

 if kept at from 10°-25° C. the colour reappeared. 



By exposing colourless cultures three months old to 

 56° and 63° C. for 5 minutes, a variety was obtained 

 which remained colourless on potatoes at all temper- 

 atures. This new variety was very feeble and may be 

 regarded as a degenerated form. 



^ Etude sur la Variability du Bacille rouge de Kiel,' Annates de Vln- 

 stitut Pasteur, vol. iv., 1890, p. 478. 



