time; and he observes, that a difference in this respect is observable in the odoriferous effluvia of vegetables. “If the air be loaded with moisture, they reach to a much greater distance. Vapour arising from a field of beans, for instance, or a putrid ditch, is sensible to the nostrils at a greater distance if the air is moist.” He observes indeed that this has never been verified with regard to infection; but as it is evidently the case with putrid effluvia, which very often accompany infection, we may reasonably conclude that it is the case with the latter also. Let us next take a view of what is advanced by the authors of The Science of Life upon this subject. Mr. McLean, who puts his name to this part, informs us of his conviction “that no general disease, which affects a person more than once during life, can ever be communicated by contagion;” and he defines contagion “a specific matter, generated in a person affected with disease, and capable of communicating that particular disease, with or without contact, to another.” It would here be no improper question, by what means he comes to know that a contagious disease can affect a person only _once_. But even this question is unnecessary. Dr. Guthrie gives an account of a gentleman who had the courage to inoculate himself for the plague, in consequence of which he had the disease with the concomitant symptoms of buboes, &c. Here then we see the plague communicated by “a specific matter generated in a person affected” with the same disease, i. e. by _contagion_, according to Mr. McLean’s own definition. The dispute therefore might stop, as this fact seems to be decisive on the subject; but as he has at great length insisted upon the argument last quoted from Dr. Mosely, it seems necessary to follow him a little farther. “If a person (says our author) be affected with any disease, it will necessarily be communicated to every other person who comes within the infectious distance, and _is not at the same time labouring under some disease higher in degree_.” This proceeds upon a supposition that his theory is absolutely perfect and infallible; which, however plain it may appear to himself, will not probably be admitted by others without some proof. Indeed he himself afterwards adduces some facts which decisively overthrow it. “A child (says he) here and there is exempted from small-pox, even though exposed to its contagion.” How comes this to pass? The disease, we are told, is contagious, the child is exposed to the contagion, and yet is not affected. In _all_ such cases it would be ridiculous to suppose the subjects labouring under a disease higher in degree than the contagion could produce. In numbers of instances of this kind the children were evidently in good health, and yet would perhaps be seized at an after period when no more exposed to contagion than they had been at first.