Dr Patrick Russell was satisfied, from the observation of much of that pestilence, that the atmospheric contagion did not extend the distance of four feet; and there is much room to think that, if he had extended his inquiry farther, that had he been aware how unusual it is for a proximate animal principle, as contagious matter, to take on the aeriform state, he would have arrived at the conclusion, that it did not only not exist, at the distance of four feet from the patient, but that it did not exist at all. Had he gone that length, he would not have created any more difficulties, to be explained away, than were made by laying down for it, such a limited range of operation, for there would, it seems, be little difficulty, in general, in discovering, that persons who had approached so near as four feet to the patient, had come in contact either with the sick himself, or the matter of the sores attached to clothes or other bodies. We know of no facts capable of proving that the matter of plague is diffusible through the air; and the very evidence of Dr Russell, which was used by him to prove the limited range of atmospheric contagion, may be used to lend countenance to the position, that it does not exist at all. The evidence was this:—Dr Russell was in the practice, at Aleppo, of examining plague sores from a window four feet from the patient, yet he suffered not from that pestilence. Scarlet Fever is a disease universally held to be one of those propagated by a contagious principle. It is commonly believed that a contagious poison is eliminated in the course of this disease, similar to that of small-pox. Its history is marked by this remarkable feature, peculiar to acute contagious diseases, of attacking the same individual only once; and the disease is accompanied by a peculiar eruption, which may, without impropriety, be supposed to contain the said contagious poison. This eruption is uniform in the time of its appearance, its duration, and decay, like the other eruptions of other contagious diseases. On all these accounts, the Author is disposed to assent to its possession of the contagious poison;—and that will be taken for granted. Connected with this view, is an observation made by Dr Sidey, of Edinburgh, in a paper contained in a late Number of the Edinburgh Medical and Surgical Journal, on Scarlatina, as lately prevalent in that town. It is to this effect, that he found that the disease, when characterized by a distinct eruption, attacked several members of a family more frequently, than when it wanted that symptom. We will inquire whether persons exposed to an atmosphere containing one sick of that disease, take that distemper as uniformly, as those take the respective diseases of those palpable contagious poisons which may be inserted under the skin.