It was about the time when the last of these advertisements appeared that Jenner commenced his inquiries concerning the prophylactic virtues of cow-pox, though nearly twenty years elapsed before they were sufficiently advanced to enable him to make the results known. His idea of using vaccine inoculation to bring about the total extinction of small pox was scouted by those of his professional brethren to whom he mentioned it, and we learn from one of his biographers that, at the outset, "both his own observation and that of other medical men of his acquaintance proved to him that what was commonly called cow-pox was not a certain preventive of small-pox. But he ascertained by assiduous inquiry and personal investigation that cows were liable to various kinds of eruption on their teats, all capable of being communicated to the hands of the milkers; and that such sores when so communicated were all called cow-pox." But when he had traced out the nature of these various diseases, and ascertained which of them possessed the protective virtue against small-pox, he was again foiled by learning that in some cases when what he now called the true cow-pox broke out among the cattle on a dairy farm, and had been communicated to the milkers, they subsequently had small-pox. These repeated failures perplexed him, but at the same time stimulated, instead of discouraging him. He conceived the idea that the virus of cow-pox might undergo some change which deprived it of its protective power, while still enabling it to communicate a disease to human beings. Following up the inquiry from this point, he at length discovered that the virus was capable of imparting protection against small-pox only in a certain condition of the pustule. He was now prepared to submit his theory to the test of experiment, but it was not until 1796 that he had the opportunity. A dairymaid, who had contracted cow-pox from one of her employer's cows, afforded the matter, and Jenner introduced it into two incisions in the arms of a boy about eight years of age. The disease thus transferred ran its ordinary course without any ill effects, and the boy was afterwards inoculated with the virus of small pox, which produced no effect. The disappearance of the cow-pox from the dairies in the neighbourhood of his country practice in Gloucestershire prevented him from making further experiments; and when he visited London for that purpose, he had the mortification of finding that no one could be found who would consent to be operated upon. It was not until 1798 that this obstacle was overcome, and then, the results of the earlier experiments having been confirmed by a series of vaccinations, followed by inoculation for small-pox several months afterwards without effect, Jenner made his discovery public.