The third proposition is of most material importance; for I am well convinced that upon a proper understanding of this question depended the true value of inoculation as far as regarded _individuals;_ and on it also rests the true value of Vaccination as regards _society in general_. It was unfortunate for the cause of inoculation that its early advocates maintained the absolute impossibility of Small Pox occurring a second time in the same individual. They thus furnished their opponents with weapons against themselves, in the cases, which repeatedly occurred, of those who had passed through the process of inoculation being afterwards affected by the natural Small Pox; and were put to the necessity of having recourse to expedients equally unmanly and uncandid—either of denying that the second disease was genuine Small Pox, or of asserting that the inoculating process had been imperfectly performed. Whereas a little attentive observation and research into the former history of Small Pox, might have satisfied them, that although in a great majority of cases this formidable disease did not occur a second time in the same person, yet that repeated instances took place wherein it did so occur. And that while it might be assumed as a general rule that the same individual would be affected once only with Small Pox—it was a rule admitting of exceptions. That a second attack of Small Pox may take place in the same individual, may be proved by a mass of evidence which appears to me to be perfectly irresistible; and as I consider it very important to the object of this enquiry that every doubt should be removed on the subject, I must be permitted to dwell on it longer perhaps than may at first sight appear necessary. I have great pleasure in acknowledging my obligation to the valuable “Historical Sketch of Small Pox,” by Professor Thomson of Edinburgh, for a large share of the evidence which I shall lay before my readers, to prove the frequency of the recurrence of Small Pox. Soon after the introduction of inoculation into France, “a son of M. Delatour, about nine years of age, was inoculated in 1756 for Small Pox, by Surgeon Martin, under the inspection of M. Tronchin, and passed through the disease in a satisfactory manner. This boy remained in good health for upwards of two years, when an eruption, supposed by some to be Small Pox, appeared upon him, as well as upon four of his companions in the same boarding-school. The different opinions formed of the nature of this eruption by the medical practitioners who saw it, and who seem to have judged of it according to the preconceived notions they entertained with regard to the possibility of the recurrence of Small Pox, present (says Dr. Thomson) so true a picture of what has occurred in similar cases since the introduction of Vaccination, and of the manner in which doubtful cases of varioloid eruptions continue still to be judged of, that I cannot