The cow-pox is a disease of the eruptive kind, which is sometimes extensively prevalent among cattle in large dairy countries where they are herded together in numbers, but often disappears for a long time together. Though commonly mild, it is occasionally so severe as to terminate fatally; and it is believed, on strong grounds, to have been at different times even pestilential among them, and as such, to have been mentioned by various writers on rural economy, ancient and modern, as well as in medical and other histories. It is generally, however, a very mild disorder, appearing on the udder of the cow, at first in the form of vesicles much resembling those of small-pox; and it is sometimes, as in the instance which first attracted the attention of Jenner, communicated to the hands of milkers. In such cases, an eruption of similar vesicles takes place on the hands and arms, not without much swelling and inflammation, and occasionally with fever and disturbance of the health for some days. It has never been known to prove fatal when thus communicated, or to have left any unpleasant effects behind it, except a few indented marks in the situation occupied by the pustules. It is not communicable, like small-pox in the human subject, by the effluvia; but the matter, or lymph as it is called, contained in the vesicles, must be actually inserted under the skin, or applied to a raw or an absorbing surface. But the most important of its peculiarities is the security it affords against the infection of small-pox. This property was well known among the agricultural classes in the grazing districts before the time of Jenner, and it has been stated that individuals among them had turned their knowledge of it to account for the protection of their families, by inoculating them with the vaccine disease. But this circumstance, alleged on very scanty evidence by those who were opposed to Jenner’s claims, cannot lessen the merit of his independent discovery, of which each step was communicated in succession to a numerous circle of medical friends, and is recorded in the most authentic form. His reputation is, on the other hand, enhanced by the fact that, although the immunity conferred by the casual disease in milkers had frequently come under the notice of medical men from their failing in such persons to produce the small-pox by inoculation, yet the idea of introducing the disease of an animal into the human frame was so little in consonance with any former practice, that Jenner was the first among his brethren to conjecture that cow-pox, as the milder disease, might advantageously supersede the inoculated small-pox; and that, as the latter is rendered less virulent by inoculation, so the former introduced in the same way might be milder than the casual complaint, and yet retain its protecting power. He had even communicated this conjecture to Hunter, himself no mean innovator in medicine, so early as the year 1770; and Mr.