But for this enfeebled microbe to be a real vaccine, comparable to that of cow-pox, must it not be fixed, so to speak, in its own variety, so that there should be no necessity for having recourse again to the preparation from which it was originally derived? Jenner, when he had demonstrated that cow-pox vaccination preserved from small-pox, feared for some time that it would be always necessary to have recourse to the cow to procure fresh vaccinating matter. His true discovery consisted in establishing that the cow-pox from the cow could be dispensed with, and that inoculation could be performed from arm to arm. Pasteur made his enfeebled microbe pass from one cultivation to another. What would it become? Would it resume its very active virulence, or would it preserve its moderate virulence? The virulence remained enfeebled and, we may say, unchanged. This showed it to be a real vaccine. Some veterinary surgeons and farmers, on the announcement of this discovery, applied to Pasteur for a vaccine against the disease which was so disastrous among their poultry. Some trials were made, and all succeeded beyond expectation. To preserve this vaccine it must be secured from contact with the air, the cultures being enclosed in tubes, the extremities of which are sealed by the flame of a blowpipe. What takes place during that interval of time intentionally placed between two successive cultivations of the cholera microbe--that interval which is employed in effecting the attenuation and producing the vaccine? What is the secret of this result? The agent which intervenes is no other than the oxygen of the air. Here is the proof. If the cultivation of this microbe is carried on in a tube containing very little air, and if the tube is then closed by the flame of a lamp, the microbe, by its development and life, quickly appropriates all the free oxygen contained in the tube, as well as the oxygen dissolved in the liquid. Thus, completely protected from contact with oxygen, the microbe does not become sensibly weakened for months, sometimes even for years. The oxygen of the air, then, appears to be the cause of modification in the virulence of the microbe. But how, then, is the absence of influence on the part of the atmospheric oxygen, in the successive cultures which are practised every twenty-four hours, to be explained? There is, in Pasteur's opinion, but one possible explanation; it is that the oxygen of the air in this latter case is solely employed in the life of the microbe. A culture has a duration of some days; in twenty-four hours it is not terminated. The air which comes in contact with it is then entirely employed in nourishing and largely reproducing the microbe. During the longer intervals of culture, the air acts only as a modifier, and at last there arrives a moment when the virulence is so much weakened as to become nil.