It would be impossible in the brief space at our disposal to recount the various difficulties and sources of error that Jenner encountered. It may, however, be mentioned that he ascertained that there was more than one form of local disease with which cows are afflicted, and which may give rise to sores on the hands of milkmaids, but that one only of these was the true cow-pox, giving origin to constitutional as well as local disease, and which proves protective against small-pox. He also found reason to believe that it was only in a particular stage of its development that the true cow-pox vesicle was capable of being transmitted so as to prove a prophylactic [preventive]. He was aware that though, as a rule, persons did not have small-pox a second time, yet there are instances where, from peculiarity of constitution or other causes, small-pox occurs a second time in the same individual. Such considerations as these cheered him to continue his inquiries when apparent exceptions occurred to the protective influence of true cow-pox. Having at length satisfied his own mind, and, indeed, succeeded in convincing others also, respecting the important protective influence exerted on the constitutions of those who had received the true cow-pox in the casual way, he sought to prove whether it was possible to propagate the disease by inoculation from one human being to another. On the 19th May, 1796, an opportunity occurred of making the experiment. Matter was taken from the hand of Sarah Nelmes, who had been infected by her master's cows, and inserted into the arm of James Phipps, a healthy boy eight years of age. He went through the disease in a regular and perfectly satisfactory way. But was he secure against the contagion of small-pox? It is needless to say how full of anxiety Jenner was, when in July following he put this to the test by inoculating the boy with matter taken from the pustule of a small-pox patient. No disease followed! This, his first crucial experiment, Jenner related to his friend Gardner, and said: "I shall now pursue my experiments with redoubled ardour." This ever-to-be-remembered day, 19th May, 1796, is commemorated by an annual festival in Berlin, where, in 1819, little more than twenty years after, it was officially reported that 307,596 persons had been vaccinated in the Prussian dominions alone. The account which Jenner has given of his own feelings at this time is deeply interesting. "While the vaccine discovery was progressing," he says, "the joy I felt at the prospect before me of being the instrument destined to take away from the world one of its greatest calamities, blended with the fond hope of enjoying independence and domestic peace and happiness, was often so excessive that in pursuing my favourite subject among the meadows I have sometimes found myself in a kind of reverie. It is pleasant to me to recollect that these reflections always ended in devout acknowledgments to that Being from whom this and all