The opinion of Dr. Cullen concerning contagion has been already noticed, p. 179; but though he supposes it to be a matter floating in the atmosphere, he observes that contagions are never “found to act but when they are near to the sources from whence they arise; that is, either near to the bodies of men, from which they immediately issue, or near to some substances which, as having been near to the bodies of men, are embued with their effluvia, and in which substances these effluvia are sometimes retained in an active state for a very long time. The substances thus embued with an active matter may be called _fomites_; and it appears to me probable, that contagions as they arise from fomites, are more powerful than as they arise immediately from the human body.” This opinion concerning the great power of contagion imbibed by certain substances is conformable to what was above laid down by reasoning _a priori_ on the nature of contagion.[98] It is doubted by Dr. Russel, but Dr. Lind adopts it, and Van Swieten gives his opinion to the same purpose. “I am convinced, that the body of the diseased, kept exactly neat and clean, is not so liable to impress the taint, as his late wearing apparel, dirty linen, and uncleanliness of any sort about him long retained in that impure state. I say, these last contain a more concentrated and contagious poison than the newly emitted effluvia or excretions of the sick.” [98] See p. 226. With regard to the original cause, our author observes, “that the plague is bred or produced originally from vitiated human effluvia, is a matter which has by no means been established on proper authority.” Setting aside therefore inquiries of this kind, he thinks it sufficient to inquire whether the infection be not communicated from a sick to a sound person by immediate contact; whether it be not also communicated at some distance through the medium of the air; and whether substances of various kinds do not imbibe the infectious effluvia, and retain them for a considerable time. So far as these points admit of proof from the experience of times past, the question concerning pestilential contagion will admit of a solution, independent of all theoretic reasoning whatever. 2. On the subject of contagion people have been embarrassed by confounding the true plague with other malignant diseases. This has been done, not only by the ancients, but by some moderns; and our author quotes Dr. Pye, saying “that any epidemic sickness, which rages with more than ordinary violence, and which occasions extraordinary mortality amongst mankind, may be, and is, properly termed a pestilence, or the plague.” By not attending to the proper distinctions, in these cases, circumstances belonging to what are commonly termed malignant or pestilential fevers will often come to be very improperly applied to the true plague.