air to its surface, and may then be said to
consist of contagious miasmata. This kind of contagious matter only induces
fever, but does not produce other matter with properties similar to its
own; and in this respect it differs from the contagious miasmata of
small-pox or measles, but resembles those which have their origin in
crowded jails; for these produce fever only, which frequently destroys the
patient; but do not produce other matters similar to themselves; as appears
from none of those, who died of the jail-fever, caught at the famous black
assizes at Oxford, at the beginning of this century, having infected their
physicians or attendants.

If indeed the matter has continued so long as to become putrid, and thus to
have given out air from a part of it, it acquires the power of producing
fever; in the same manner as if the ulcer had been opened, and exposed to
the common air; instances of which are not unfrequent. And from these
circumstances it seems probable, that the matters secreted by the new
vessels formed in all kinds of phlegmons, or pustles, are not contagious,
till they have acquired something from the atmosphere, or from the gas
produced by putrefaction; which will account for some phenomena in the lues
venerea, cancer, and of other contagious secretions on the skin without
fever, to be mentioned hereafter. See Class II. 1. 4. 14.

The theory of contagion has been perplexed by comparing it with fermenting
liquors; but the contagious material is shewn in Section XXXIII. to be
produced like other secreted matters by certain animal motions of the
terminations of the vessels. Hence a new kind of gland is formed at the
terminations of the vessels in the eruptions of the small-pox; the animal
motions of which produce from the blood variolous matter; as other glands
produce bile or saliva. Now if some of this matter is introduced beneath
the cuticle of a healthy person, or enters the circulation, and excites the
extremities of the blood-vessels into those kinds of diseased motions, by
which it was itself produced, either by irritation or association, these
diseased motions of the extremities of the vessels will produce other
similar contagious matter. See Sect. XXXIII. 2. 5. and 9. Hence contagion
seems to be propagated two ways; one, by the stimulus of contagious matter
applied to the part, which by an unknown law of nature excites the
stimulated vessels to produce a similar matter; as in venereal ulcers,
w