It was a persuasion among the farmers that pox on the cows was derived from grease on the horse; and that infection with horsegrease was just as good against smallpox as infection with its derivative cowpox. The fact, however, of this derivation of cowpox from horsegrease was contested, but Jenner was positive. Writing in 1794 he said-- At our last meeting our friend treated my discovery of the origin of cowpox as chimerical. Farther investigation has convinced me of the truth of my assertion beyond the possibility of a denial.[95] Challenged to produce direct evidence that grease from the horse produced pox in the cow, he met with considerable difficulty, so that on 2nd August, 1797, he had to write-- The simple experiment of applying the matter from the heel of the horse, in its proper state, to the nipples of the cows, when they are in a proper state to be infected by it, is not so easily made as at first sight may be imagined. After waiting with impatience for months in my own neighbourhood, without effect, I sent a messenger to Bristol, in vain, to procure the true virus. I even procured a young horse, kept him constantly in the stable, and fed him with beans in order to make his heels swell, but to no purpose.[96] In the matter of horsegrease, it is to be observed as Dr. Mason Good informs us, “that for ages blacksmiths and farriers, who had been infected with grease, were considered as generally insusceptible of variolous contagion.”[97] Wherefore, to Jenner is not to be ascribed the discovery of horsegrease as good against smallpox; but merely that he held with certain farmers that it was the cause of cowpox, and one in constitution with cowpox; and thus endeavoured to combine the tradition of the stable with that of the dairy. It was not until 1796 that Jenner made any experiment with cowpox--up to that date, whatever his visions, he was in Hunter’s phrase a _thinker_, not a _trier_. On 14th May of that year, he took matter from the hand of Sarah Nelmes, who had been infected by her master’s cows, and inserted it by two incisions in the arm of James Phipps, a child of eight years of age. The boy went through the disease in a regular manner, and on the 1st July was inoculated with smallpox without effect, to Jenner’s intense satisfaction. He communicated the event to Gardner in the following letter-- BERKELEY, 19th July, 1796. DEAR GARDNER,--As I promised to let you know how I proceeded in my inquiry into the nature of that singular disease the Cowpox, and being fully satisfied how much you feel interested in its success, you will be gratified in hearing that I have at length accomplished what I have been so long waiting for, the passing of the Vaccine Virus from one human being to another by the ordinary mode of inoculation.