are these “Ansteckungen!”, from the purely mental, on the one hand, which, through the mere sight of a disagreeable nervous malady—through an excitement of the senses that shakes the mind, penetrates into the nerves, those channels of its will and of its feelings, and produces the same disorder in the beholder, to those, on the other hand, which propagate diseases that principally operate only upon matter, and are distinguishable but little, if at all, from animal poisons. The reader must not here expect all the features of a doctrine which extends through the whole immeasurable domain of life. They are clearly derived from the confirmed and well applied experience of _the past_, and have been delineated by men[470] who had not forgotten, like their modern successors, to take a comprehensive view of epidemic diseases. It may, however, be permitted me just to call to mind the difference between those infectious diseases which are _permanent_ and for centuries together _unchangeable_, and those which are _temporary_ and _transient_. The infecting matter of the former may aptly be called the perfect or unchangeable in contradistinction to the imperfect or mutable character of the latter. The former, when once formed, whether in diseased persons or inanimate substances (fomites), are always in existence, and are but called into activity by those causes of general disease (epidemic constitutions) which are favourable to their propagation; and it is to be remarked that under all circumstances, and at all times, they excite the same unchangeable diseases, and, varying only in particular ramifications or degenerations and mild forms, never lose their proper essence. Examples are furnished in the small-pox, the plague, the measles, and, if we may include diseases not febrile, the leprosy, the itch, and the venereal disease. The latter, on the other hand, are not always in existence, they are called forth from nonentity by the causes of general diseases or epidemic constitutions, and they disappear again after the extinction of the epidemic diseases by which they were bred; they likewise vary in their development and their course in each particular epidemic. Examples are found in the yellow fever, in catarrh or influenza, in nervous and putrid fever, and, among many other disorders, in miliary fever, a disease which first grew to a national pestilence in the 17th century, and which, in the kind and manner of its infecting power, approaches nearest to the sweating fever. To this latter category the English Sweating Sickness likewise belongs; a disease altogether of a temporary character, which, after its cessation, left no infecting material behind, and consequently was incapable of propagating itself after the manner of those diseases which are completely contagious. The animal matters, which were expelled along with the profuse perspiration, and spread so horrible a stench around the sick, contained amid their alkaline salts, (probably