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Jeiiner owed to a popular rumour his first idea of the protective power of cow-pox. By the way of experiment he established its truth. By experi- ment also he proved, that the vaccine virus could be transmitted from one human being to another and still retain its pristine virtue, and so could be provided fresh for all times and seasons, indepen- dently of the casual outbreaks of the disease among cows. INIoreover, he found out, and laid down clearly, rules for the practice of Vaccination, the observance of which was, and is, essential to its success. And, in justice to Jenner, it must not be forgotten, that the idea of permanently and harmlessly and specifically modifying the constitu- tion of a man by imparting to him the disease of a beast was an idea entirely new to science. Perhaps we may draw some moral from the fact, that a man unobtrusive and unambitious, and not distingiushed above his contemporaries for the powers of his genius, did yet become one of the greatest benefactors of his race. It was not, that . to Jenner s ears alone the popidar rumour had reached, that the cow-pox was a protection against the small-pox. Many and able men had heard of ii{n). Nor was it merely the opportunities af-