ON THE BACTERIA. 159 



things will remain in this condition for a longer or shorter time, 

 even for months, without any visible alteration, provided atmos- 

 pheric germs are excluded by a plug of cotton-wool. If we now 

 take one of these cultures — say the hundredth or the thousandth 

 for instance — and with a drop of it inoculate a certain number 

 of fowls, it will be found that it is just as fatal as if they were 

 inoculated with a drop of blood taken directly from a fowl that 

 had died of the disease. They will all die equally quickly, and 

 with the same symptoms, and their blood after death will be 

 found to contain the same minute organisms. But if, instead of 

 repeating our cultures at periods of only a few days, we make 

 them at intervals of one, two, four, six, or even ten months, we 

 shall find that the virulence of the successive cultures is greatly 

 altered ; and that, if we now repeat our experiments of inoculating 

 healthy fowls with our cultivated fluids, we shall discover that 

 one -preparation will be fatal to eight out of ten, another to five 

 out of ten, and another to none at all, although the Bacteria 

 may still be cultivated, and the ten will all suffer from a modified 

 attack of the disease. Finally, if we take these attenuated 

 cultures, and without any interval, start from them fresh cultiva- 

 tions, we shall find that each of these new cultivations will retain 

 the same amount of virulence as the one from which it was 

 started. This brings us to what is practically the most important 

 point of these investigations, namely, that when the fowls have 

 been made sufficiently ill by the attenuated virus, they may after- 

 wards be inoculated with the most virulent, and will suffer no evil 

 effects, or only effects of a passing character. 



The question naturally suggests itself: what is the cause of 

 this diminution of the virulence ? Pasteur's answer to this inquiry 

 is that it depends on the effect of the oxygen of the air. If we 

 carry on our culture in a tube containing very little air, and then 

 hermetically seal it, we find that the development of the virus 

 continues until all the oxygen is exhausted ; the cloud then falls, 

 the virus is deposited on the sides of the tube, and on opening the 

 tube at the expiration of from one to ten months, we find the 

 virulence always identical with the virulence of the original culture 

 used in the preparation of the tubes ; while if some of the tubes 

 prepared at the same time, and from the same culture, are kept 



