Something indeed of this kind would seem really to be the case, otherwise we cannot well conceive why there should be such a distinction of diseases. Thus the infection of the small-pox is the same all over the world. The variolous matter will never produce the measles in any country, nor will the typhus produce a pleurisy. The plague manifests itself to be the same distemper in all its various degrees of malignity, though even this dreadful disease is sometimes so mild that it does not confine the patient to his bed. There must therefore be some certain constitution in the nature of the cause which produces such and such diseases, as certainly as in the seed of particular vegetables, which gives to each its proper appearance and shape. The cause of the disease so modified we may call, with Dr. Cullen, its _specific contagion_. Having thus treated so largely upon contagion of different kinds, it now remains to consider the objections that have been made to the doctrine altogether. It is indeed surprising that in so great a length of time, after the world hath so often and so dreadfully suffered from the violence of plagues, the simple fact, whether it be infectious or not, should not have been determined: nay, that it should still be questioned by physicians of no mean reputation whether such a thing as contagion or infection _can possibly exist_. Dr. Mosely in his treatise on tropical diseases treats the whole doctrine of contagion with the utmost contempt; calling it “a field for speculation, which has long amused the pedantry of the schools, and should never be entered into by practical writers.” Notwithstanding this, however, he doth enter into it, and with such bad success, that in the very first paragraph he is obliged to derive the cause of diseases from the stars! “There are some diseases we know, (says he) which follow the changes of the atmosphere; but there are others which make their revolutions, and visit the earth, at uncertain periods; for which we can trace no cause, depending on combinations, in which, perhaps, the _influence of the planets_ may have some share.” Here we have a still wider field for speculation than even _the schools_ have given us; for the Doctor ought to remember that the influence of a _planet_, producing a disease, is as truly contagion as the effluvia of a _dunghill_; and if we have a wide field to traverse when tracing it through the earth, we have one infinitely more extensive in pursuing it through the heavens. But we may be assured that planetary influence _does not_ produce diseases; for, if it did, they would in all times of pestilence overspread the face of the earth, as the influence of the planets, if they have any, certainly does. The arguments used by this author against _terrestrial_ contagion are,