“By contagion I would understand those diseases with the origin of which we are now unacquainted, but which at present can only be propagated by contact with a person, or matter from a person under similar disease. Contagious diseases, which it is now our business to consider, may be divided into chronic and acute, of the former are the itch, and several others. These are for the most part incurable by the unassisted powers of the constitution. The acute of which are the small-pox, and many other exanthemata, (these are those diseases accompanied with fever) marked with a peculiar eruption, and that attack only once, such as measles, and scarlet fever produce a critical fever, which ceases with the disease. “The chronic may attack a person as often as he is exposed to the exciting cause, the acute, for the most part, leave the constitution no longer susceptible of their operation.” After pointing out the modes of communication of contagious diseases by contact and by fomites, he says, “Infectious diseases, on the contrary, may be traced in their origin, and do not require for their production matter similar to their effects, but may at any time be generated by crowding together the sick or wounded of any description. Of this kind are the hospital, prison, or ship-fever, camp dysentery, and some peculiarly malignant ulcers. Though these diseases, when formed, may produce their like in others, yet we can always trace their origin to causes different from their effects.” From the London Cyclopædia the following extract is taken. “There does not appear to be any distinction commonly made between contagious and infectious diseases.” This extract proves how much confusion there exists, with the terms infectious and contagious. Here they are said to be used synonimously, and in that of Dr Bateman just quoted, a great distinction is drawn. Such are a few of the facts connected with the history of contagion, which are most worthy of notice, in a work of this kind. This sketch will afford some idea of the most rational views which have been, and still are, held on the subject; and of the light in which it is at present regarded by the medical world. It is feared that the extracts which have been given, may appear too copious, but it has been thought highly proper, that the opinions of those justly considered, the greatest authorities on the subject, should be given: and that they might not be misunderstood, they have been, for the most part, presented verbatim. CHAPTER IV. THE ABSENCE OF SUFFICIENT EVIDENCE OF THE EXISTENCE OF ATMOSPHERIC CONTAGION. Ponderable bodies are endowed with common or general properties, and likewise with particular or secondary properties. MAGENDIE.