Meanwhile the discovery that is associated with his name had been slowly maturing in his mind. When only an apprentice at Sodbury, his attention had been directed to the relations between cow-pox and small-pox in connexion with a popular belief which he found current in Gloucestershire, as to the antagonism between these two diseases. During his stay in London he appears to have mentioned the thing repeatedly to Hunter, who, being engrossed by other important pursuits, was not so strongly persuaded as Jenner was of its possible importance, yet spoke of it to his friends and in his lectures. After he began practice in Berkeley, Jenner was always accustomed to inquire what his professional brethren thought of it; but he found that, when medical men had noticed the popular report at all, they supposed it to be based on imperfect induction. His first careful investigation of the subject dated from about 1775, and five years elapsed before he had succeeded in clearing away the most perplexing difficulties by which it was surrounded. He first satisfied himself that two different forms of disease had been hitherto confounded under the term cow-pox, only one of which protected against small-pox, and that many of the cases of failure were to be thus accounted for; and his next step was to ascertain that the true cow-pox itself only protects when communicated at a particular stage of the disease. At the same time he came to the conclusion that "the grease" of horses is the same disease as cow-pox and small-pox, each being modified by the organism in which it was developed. For many years, cow-pox being scarce in his county, he had no opportunity of inoculating the disease, and so putting his discovery to the test, but he did all he could in the way of collecting information and communicating what he had ascertained. Thus in 1788 he carried a drawing of the cow-pox, as seen on the hands of a milkmaid, to London, and showed it to Sir E. Home and others, who agreed that it was "an interesting and curious subject." At length, on the 14th of May 1796, he was able to inoculate James Phipps, a boy about eight years old, with matter from cow-pox vesicles on the hand of Sarah Nelmes. On the 1st of the following July the boy was carefully inoculated with variolous matter, but (as Jenner had predicted) no small-pox followed. The discovery was now complete, but Jenner was unable to repeat his experiment until 1798, owing to the disappearance of cow-pox from the dairies. He then repeated his inoculations with the utmost care, and prepared a pamphlet (_Inquiry into the Cause and Effects of the Variolae Vaccinae_) which should announce his discovery to the world. Before publishing it, however, he thought it well to visit London, so as to demonstrate the truth of his assertions to his friends; but he remained in London nearly three months, without being able to find any person who would submit to be vaccinated. Soon after he had