128 TEXT-BOOK OF BACTERIOLOGY. 



tent when passed several times through the bodies of rabbits, as 

 lias been proved by Pasteur and Kitt. 



Chauveau robbed the anthrax bacilli of their poisonous property 

 by breeding them under a pressure of eight atmospheres, and 

 Arloing found that sunlight is capable of weakening these same 

 bacilli, and even their spores. 



By far the surest and most commonly-used procedure is- the 

 exposure of the micro-organisms to the influence of high tempera- 

 tures. Toussaint kept blood containing anthrax bacilli for ten min- 

 utes at 55° C. The bacilli were by no means killed by the heat, yet 

 they were rendered harmless by it. Pasteur, for his experiments 

 on a large scale, employed a considerably lower degree of heat, but 

 he has not given full particulars of his method, so that no definite 

 judgment can be formed with regard to it. We therefore owe our 

 thanks to Koch and his coadjutors, who once more approached this 

 question in a methodical, strictly scientific course of experiments, 

 with a view to ascertain the effects of heat in diminishing bacterial 

 virulence by studying the anthrax bacillus — the best known and 

 most suitable species for this purpose. 



Koch, Gaffky, and Loffler found that at 42° C. and 63° C. a 

 diminution of virulence was perceptible in the cultures. They fur- 

 ther discovered the important fact that the lower the temperature 

 is by which a diminution takes place, the longer it takes for such 

 diminution, but at the same time the more permanent are its effects. 



Even variations of a fraction of a degree are here important. 

 While anthrax bacilli can be rendered perfectly harmless in nine 

 days with 43° C, it requires twenty-four days if we only employ 

 42.6° C, but in this latter case the new quality of the bacteria has 

 become so thoroughly a second nature to them that they cannot 

 throw it aside again. They not only keep it throughout their own 

 life, but they even transmit it to their progeny. In fact, we can 

 breed from them as many generations as we will of fully harmless 

 bacteria. 



If we endeavor to diminish virulence under the influence of 

 higher temperatures more quickly — in a few days — the bacteria 

 regain it with proportional rapiditj"^. 



But their virulence cannot be destroyed with one blow. Before 

 the micro-organisms part with it completely they pass through a 

 number of intermediate stages, each of which, with the amount of 

 virulence still remaining, is sufficient to affect certain animals, the 

 efficacy of the poison remaining longest for those most susceptible 

 to it. Bacilli of twenty days at 43.6° C, for instance, will still kill 

 mice, those of twelve days will still kill Guinea-pigs, those of ten 



