“Small-pox, measles, and other general diseases, which occur only once during life, never disappear, until _the whole_ of those who have been within the infectious distance, and were not at the time labouring under some disease higher in degree, have received the infection. As these diseases are _very mild_, children sometimes resist the power of contagion from the superior force of some other diseases, although they may be so slight as to escape _common_ observation.” In this paragraph we have the favourite maxim of our author repeated, _twice_ indeed, without a single fact to support it. Instead of this we find hypothesis heaped upon hypothesis, as the giants are said to have heaped mountains upon one another in order to get up to heaven. He first supposes that the infection of the small-pox seizes on _the whole_ of those on whom it falls. The exceptions to this maxim he explains by another _supposition_, viz. that the contagion of the small-pox is counteracted by another disease. The second hypothesis is supported by a third, and that a very extraordinary one, that the small-pox (a disease which has destroyed innumerable multitudes) is _very mild_; and this third by a fourth, that the diseases which counteracted the contagion were so slight as to escape _common_ observation. It was incumbent on Mr. McLean to have pointed out some of those diseases, and to have informed us how they came to counteract this contagion. But it is needless to argue with one who writes so extravagantly. Far from the mode of reasoning followed by Dr. Fordyce, who decided from the majority of facts, our author determines every thing by his own preconceived opinions. “That the power which occasioned disease at the Oxford assizes (says he) was not contagious matter, is proved by its producing diarrhœa in some, while it produced fevers in others.” But, if it was not contagious matter, what kind of matter was it? Or how comes our author to know that those who were affected by the diarrhœa were not likewise affected by fever? How many fevers are attended by diarrhœa, or how many cease when diarrhœa comes on! It would have been equally conclusive to say that the matter was not contagious, because some died and some recovered.