Perhaps, in reference to this contagious matter and to others, it may be said that they were not favourably situated for acting. Heat, moisture, and the passing to and fro, of air, must certainly assist the assumption of the aeriform state; and a more favourable opportunity cannot be obtained, than the contagious matter of small-pox pustules has, in the mouth of the patient, where it almost always is observed. That situation is perhaps even more favourable than that of the matter operated on by Dr O’Ryan. Yet it is known, (and we are prepared to shew cases) that persons liable to the disease have breathed in the same apartment, and have not taken the distemper. We know, too, of many cases, where persons have been attacked under such circumstances, but that has probably arisen from actual contact with the matter, or exposure to those general and widely-spread influences productive of that pestilence, that undoubtedly exist. But it is not necessary for our purpose, that all should escape, but, that any should not suffer. It is enough that those who escape, are more, in proportion, than those who resist the action of the palpable poison, when inserted into the system by inoculation. With respect, also, to the disease produced by the insertion of cow-pox matter, or, in other words, by vaccination, as it is called; nobody ever heard of it being propagated through the air. It is feared that it would be a very inefficient mode of vaccinating, to bring the child to be vaccinated, into an atmosphere, to which was exposed an arm with a cow-pox. He who would propose such a plan would be laughed at by every old woman; and what is held as so absurd and ridiculous in respect to cow-pox, cannot be very wise in reference to small-pox, plague, scarlet fever, and the like. There are other diseases, too, which undoubtedly are propagated by palpable contagious poisons. Yet were any person affected with them, to whisper, that a contagious atmosphere had been the occasion, they would be held as using no small liberty with the credulity of the medical adviser. There is yet another palpable contagious matter to which reference must be made,—that of itch. The only known way by which that disease can be propagated, from one to another, is by palpable or contactual contagion. Many medical men are in the daily practice of seeing and examining such cases, yet they seldom or never are affected with it. Any caution directed against the operation of that contagion, is addressed exclusively to contact, never to the atmosphere. The plague, according to the very best authorities, is undoubtedly marked by the elimination of a matter capable of producing the same pestilence, when applied in a palpable form, to the body of another. The plague has been produced intentionally by inoculation, and may be propagated at pleasure.