What was requisite for success, he concluded, was the limpid fluid from the Horse’s heel at an early stage of the disease, and that it should be applied to the Cow’s nipples at a certain season-- The virus from the Horses’ heels is most active at the commencement of the disease, even before it has acquired a pus-like appearance; indeed I am not confident whether this property in the matter does not entirely cease as soon as it is secreted in the form of pus. I am induced to think it does cease, and that it is the thin darkish-looking fluid only, oozing from the newly formed cracks in the heels, similar to what sometimes appears from erysipelatous blisters, which gives the disease. Nor am I certain that the nipples of the Cows are at all times in a state to receive the infection. The appearance of the disease in the spring and the early part of the summer, when they are disposed to be affected with spontaneous eruptions so much more frequently than at other seasons, induces me to think, that the virus from the Horse must be received upon them when they are in this state in order to produce effects. Experiments, however, must determine these points. (P. 45.) Whilst thus explicit as to what was requisite for the infection of the Cow by the Horse, Jenner did not succeed in producing Cowpox from Horsegrease. He had to write-- The spring of the year 1797, which I intended particularly to have devoted to the completion of this investigation, proved from its dryness remarkably adverse to my wishes. No Cowpox appeared in the neighbourhood; for it most frequently happens that while the farmers’ Horses are exposed to the cold rains of spring their heels become diseased. (P. 44.) Yet without proof, he argued as if he had proof, saying-- With respect to the opinion adduced, that the source of the infection is a peculiar morbid matter arising in the Horse, although I have not been able to prove it from actual experiments conducted immediately under my own eye, yet the evidence I have adduced appears sufficient to establish it. (P. 43.) Evidence adduced! Of evidence there was none. The farmers might be right in their opinion that Cowpox sprang from Horsegrease, but opinion was not evidence, nor even such assurance as this of Jenner’s-- I feel no room for hesitation respecting the common origin of the disease, being well convinced that it never appears among the Cows unless they have been milked by some who at the same time has the care of a Horse affected with diseased heels. (P. 44.) But not even to this conviction did he adhere. “It was highly probable,” he thought, “that not only the heels of the Horse, but other parts of the body of that animal, are capable of generating the virus which produces the Cowpox”--