The preceding reasoning is grounded on the supposition of extirpation: but, however demonstrable the _possibility_ of extirpation may be, it may not in all communities be _practicable_; and may not these alleged failures so operate, as, in such circumstances, to render the expediency of the practice questionable? In order to decide this, let the nature and amount of these failures be ascertained and estimated. The description of those cases of Small Pox, (if they can be called so,) which occur in vaccinated subjects, is shortly as follows:—The invasion and eruption in every respect resembles that of the genuine Small Pox. I have seen it attended with high fever and a thick crowded crop of _papulæ_, such as precedes the most severe and dangerous cases of the confluent kind. This runs on till the fifth day from the eruption, both days included, at which time some of the _papulæ_ begin to be converted into small sized pustules. The disorder then abruptly stops short. On the following day the fever is found to have subsided, with a shrivelling and desiccation of the eruption, and recovery proceeds without the least danger or inconvenience. The face is marked, for some time after, with brown spots, but without pits. It should never be forgotten, that all morbid _phænomena_ are full of varieties and exceptions. Accordingly, though the fifth day is the most common limit of this disorder, it sometimes stops short on the third; sometimes not till the sixth or seventh; and, in a very few cases, it has been known to run the common course of Small Pox. What forms the strong line of distinction from proper Small Pox, is that, with a few exceptions, it does not advance to maturation and secondary fever, which is the only period of danger. I am not prepared to deny that death may not have occurred in a few instances; nay, there seems sufficient evidence that it actually has; but these adverse cases are so rare, as not to form the shadow of an objection to the expediency of the general practice. A few weeks ago at a meeting of this Society, at which forty members and visitors were present, I put the question whether any of these eminent and extensive practitioners had met with any fatal cases of this kind. Two gentlemen had each seen a single case, and two other gentlemen took occasion to say that they had each seen a case of second Small Pox, both of which proved fatal. It is evident, therefore, that according to that maxim which guides mankind in the conduct of life, namely, that of acting on a general rule and average, and not on exceptions, these adverse instances ought not to have the least influence on practice, even though they were much more numerous. Nor indeed do they, except in the very rare cases here cited, deserve the name of failures; for, though they fail in preventing _Small Pox_, they do not fail to prevent _Death_. And let me here, in the name of humanity, beseech practitioners