It may be interesting to relate that this important discovery was made unintentionally, and represents one of those happy ‘accidents’ which occur to those who diligently search. Pasteur had been working with cultures of chicken-cholera microbes, an extremely fatal form of virus when it is introduced into fowls and small birds. It so happened that one of his cultures was left forgotten in the incubator when work was stopped for the vacation. On the return of Pasteur and his assistants the experiments were continued. When the bottle was discovered, thinking that the microbes might have been exhausted or dead from long starvation, Pasteur tried to make what is called a fresh culture of them, by inseminating a sample from the old bottle into a freshly prepared nutritious broth. The microbes were not dead, and multiplied and grew luxuriantly; but when they were injected into a fowl they caused only a transient and non-fatal disease. To make a fresh start, Pasteur took some old blood, which he had drawn a long time previously from a chicken-cholera fowl and preserved in a cupboard in the laboratory in a sealed-up tube, and made a culture with the material that was in that tube. The culture thus obtained killed fresh fowls as usual, but when it was injected into the bird that had resisted the first culture it resisted this injection also. Pasteur, who excelled all men I ever knew in his ability of quickly analyzing and discerning true connections between facts, required no further hints. Others might perhaps have dwelt on the peculiarity of the fowl that happened to resist the injections, or on some other circumstances. Pasteur relinquished this and other suggestions at once, and thought of the microbe. The fact that old specimens of microbes may become impotent when injected into animals was known to him, and was readily explained by the vitality of such microbes being lowered or exhausted by starvation. But, then, such a microbe when transferred into a fresh medium, if not dead, generally regains its vigor, and after that, when inoculated into an animal, it produces its usual effect. The remarkable circumstance about the culture left in the incubator was that even when it was transferred into a fresh medium and its vitality renewed, it remained still impotent. Pasteur concluded from this that an infectious microbe possesses two distinct properties: one, which it shares with any other living being--viz., _vitality_--which may be weakened or strengthened according to the conditions of life and food; and another, which he considered as its ‘virulence,’ its power of causing diseases, which may be also weakened or strengthened by special means, but which is quite independent of ‘vitality.’