METHODS OF CULTIVATION. 169 



The earlier experimenters assumed that a boiling 

 temperature must be fatal to the minute organisms 

 developed in organic infusions; and this false as- 

 sumption furnished a foundation for the belief, 

 entertained by some of them, that bacteria might 

 appear in such fluids by heterogenesis. The as- 

 sumption has been proved to be false by the 

 experiments of Pasteur, of Tyndall and of many 

 others, and it is now known that the reproductive 

 spores, of endogenous formation, which are devel- 

 oped in certain species, may resist a temperature 

 considerably above the boiling-point of water. 

 (See p. 119.) The writer, while conducting a se- 

 ries of experiments in the biological laboratory of 

 Johns Hopkins University, during the summer 

 of 1881, was greatly troubled by the fact that the 

 laboratory was infected by the spores of a species 

 of bacillus, which developed in little islands on the 

 surface of his culture-fluids, even when they had 

 been boiled for an hour or more. To destroy the 

 spores of this bacillus, it was necessary to resort to 

 the use of a bath of paraffine, or of concentrated 

 salt-solution, by means of which a temperature of 

 105 C. was secured. This temperature, main- 

 tained for half an hour to an hour, proved effec- 

 tual in the destruction of these ubiquitous spores. 



Prolonged boiling will doubtless destroy the vi- 

 tality of the most refractory spores ; but the exact 

 time which is required to secure success in every 

 case has not been determined. In practice, it will 

 be found best to keep on the safe side, as the loss 



