96 MICROBES AND TOXINS 



be better to speak of the lethal zone and of the zone of 

 coagulation. 



It was from the effects of heat on the bacilli that Pasteur 

 discovered the anthrax vaccines. 



Microbes stand low temperatures very well. Long ago 

 Cagniard de la Tour observed that yeast kept at - 90° C. in 

 a mixture of carbonic acid and ether does not lose its power 

 of fermentation. After twenty hours at - 130° C, 108 hours 

 at — 70° C, the spores of B. subtilis still germinate, and 

 the spores of anthrax are still virulent. According to 

 MacFadyen's experiments, bacteria kept for six months at the 

 temperature of liquid air (about - 190" C.) or ten hours at 

 the temperature of liquid hydrogen ( — 252° C.) remain living 

 and virulent. 



Action of Light. — Light is injurious to bacteria and is 

 thus a disinfectant. 



The active rays are the chemical rays of the spectrum which 

 act by oxidizing the protoplasm : the bacteria do not die when 

 the sunlight strikes them in a vacuum. 



The anthrax spores stand sunlight for about thirty hours 

 in contact with air and eighty hours when shut off from air 

 (Roux). Even in vacuo in pure hydrogen the bacteria do not 

 resist indefinitely. There is therefore something else than 

 simple oxidation taking place. The action of the air is 

 associated with an action belonging more particularly to the 

 light, and the oxidation affects not only the bacterium but the 

 medium in which it is. 



The bactericidal rays are par excellence the ultra-violet 

 rays, as can be proved by cutting off certain parts of the 

 spectrum by means of various sorts of screens. Glass of a 

 thickness of i"35 millimetres completely abolishes the action. 

 A solution of oxalic acid of 10 per cent., which limits the 

 spectrum up to 300/x/t, acts in the same way, whereas bacteria 

 are destroyed through a screen of sulphocyanide of potassium 

 of 10 per cent, which cuts down the spectrum to 265/x/x 

 (experiments with an electric arc) ; the active portion of the 

 rays of such an arc must lie between these limits. A blue 



