unts for the opinion, that the pulmonary consumption is sometimes
infectious, which opinion was held by the ancients, and continues in Italy
at present; and I have myself seen three or four instances, where a husband
and wife, who have slept together, and have thus much received each other's
breath, who have infected each other, and both died in consequence of the
original taint of only one of them. This also accounts for the abscesses in
various parts of the body, that are sometimes produced after the inoculated
small-pox is terminated; for this second absorption of variolous matter
acts like common matter, and produces only irritative fever in those
children, whose constitutions have already experienced the absorption of
common matter; and inflammation with a tendency to produce new abscesses in
those, whose constitutions have not experienced the absorptions of common
matter.

It is probable, that more certain proofs might have been found to shew,
that common matter is infectious the first time it is absorbed, tending to
produce similar abscesses, but not the second time of its absorption, if
this subject had been attended to.

8. These contagious diseases are very numerous, as the plague, small-pox,
chicken-pox, measles, scarlet-fever, pemphigus, catarrh, chincough,
venereal disease, itch, trichoma, tinea. The infectious material does not
seem to be dissolved by the air, but only mixed with it perhaps in fine
powder, which soon subsides; since many of these contagions can only be
received by actual contact; and others of them only at small distances from
the infected person; as is evident from many persons having been near
patients of the small-pox without acquiring the disease.

The reason, why many of these diseases are received but once, and others
repeatedly, is not well understood; it appears to me, that the constitution
becomes so accustomed to the stimuli of these infectious materials, by
having once experienced them, that though irritative motions, as hectic
fevers, may again be produced by them, yet no sensation, and in consequence
no general inflammation succeeds; as disagreeable smells or tastes by habit
cease to be perceived; they continue indeed to excite irritative ideas on
the organs of sense, but these are not succeeded by sensation.

There are many irritative motions, which were at first succeeded by
sensation, but which by frequent repetition cease to excite sensation, as
explained in Sect. XX. on