A little drop of splenic fever blood, sown in urine or in the water of yeast, previously sterilised--that is to say, rendered _un_putrescible by contact with air free from all suspended germs--produces in a few hours myriads of bacilli or of bacteria. A little drop of this first cultivation sown in a second flask containing the same liquid as the first and prepared with the same precautions as to sterility and purity, shows itself no less fertile. Finally, after ten or twenty similar cultures the parasite is evidently freed from the substances which the initial drop of blood might carry with it; yet, if a very small quantity of the last culture is injected under the skin of a rabbit or a sheep, it kills them in two or three days at most, with all the clinical symptoms of natural splenic fever. It might be objected that the parasite was associated in the cultivating liquid with some dissolved substance that it had produced during its life and which acted as a poison. Pasteur accordingly transported some cultivating tubes into the cellars of the Observatory, where a temperature absolutely constant reigned, a circumstance which permits of the deposit of all the parasitic filaments at the bottom of the tubes. Inoculating afterwards both with the clear upper liquid and with the deposit at the bottom, he found that the latter alone produced disease and death. It is, then, the bacteria which cause splenic fever. The proof was given and no further doubt remained. I. Yes, splenic fever is no doubt produced by bacteria just as itch is produced by acaries and trichinosis by trichinæ. The only difference is that the parasite of splenic fever can only be seen by means of a rather powerful microscope. Here, then, is a disease in the highest degree virulent, due in its first cause to the _infinitely little_. Pasteur laid hold of and isolated this terrible virus. It was in a microscopic parasite, and in it alone, that the virulence of splenic fever resided. A great scientific fact had been gained. A virus might consist not of amorphous matter, but of microscopic beings. The virulence was due to their life. Liebig, and all the chemists and doctors who had accepted and maintained his doctrine, totally repudiated all vital action in fermentation as well as in contagious and infectious diseases. Dominated by their hypotheses, they allowed themselves to be deceived by false assimilations to facts of a purely chemical kind, which appeared to them to be connected with the phenomena of fermentation and virulence. Liebig wrote, 'By the contact of the virus of small-pox the blood undergoes an alteration, in consequence of which its elements reproduce the virus, and this metamorphosis is not arrested until after the complete transformation of all the globules capable of decomposition.'