This impediment to my progress was not long removed before another, of far greater magnitude in appearance, started up. There were not wanting instances to prove that when the true Cowpox broke out among the cattle at a dairy, the person who had milked an infected animal, and had thereby gone through the disease in common with others, was liable to receive the Smallpox afterwards. This, like the former obstacle, gave a painful check to my fond and aspiring hopes; but reflecting that the operations of Nature are generally uniform, and that it was not probable the human constitution (having undergone the Cowpox) should in some instances be perfectly shielded from the Smallpox, and in many others remain unprotected, I resumed my labours with redoubled ardour. The result was fortunate; for I now discovered that the virus of Cowpox was liable to undergo progressive changes from the same cause precisely as that of Smallpox, and that when it was applied to the human skin in its degenerated state, it would produce the ulcerated effects in as great a degree as when it was not decomposed, and sometimes far greater; but having lost its _specific properties_, it was incapable of producing that change upon the human frame which is requisite to render it insusceptible of the variolous contagion: so that it became evident a person might milk a Cow one day, and having caught the disease, _be for ever secure_; while another person, milking the same Cow the next day, might feel the influence of the virus in such a way as to produce a sore, or sores, and, in consequence of this, might experience an indisposition to a considerable extent; yet, as has been observed, the specific quality being lost, the constitution would receive no peculiar impression.... This observation will fully explain the source of those errors which have been committed by many inoculators of the Cowpox. Conceiving the whole process to be extremely simple, as not to admit of a mistake, they have been heedless about the state of the Vaccine Virus; and finding it limpid, as part of it will be, even in an advanced state of the pustule, they have felt an improper confidence, and sometimes mistaken a spurious pustule for that which possesses the perfect character. No one apparently thought it worth while to expose the fictitious character of these statements, invented by Jenner to justify his pretensions and to baffle objections. Any careful reader of the _Inquiry_ of 1798, and the _Origin of Vaccine Inoculation_ of 1801, cannot fail to perceive the radical inconsistency of the earlier and later narratives, and how a few hasty experiments enveloped in unverifiable conjecture and gossip, came to be magnified into years of arduous research. He wound up his statement with this flourish and prediction--