The bent of Sydenham’s mind was eminently practical; he thought that the business of a physician is to acquire an accurate knowledge of the causes and symptoms of diseases, and the effects of different remedies upon them, that if he cannot prevent them, he may at least recognise them with certainty, and apply with promptitude the means most likely to cure them: with Hippocrates and the ancient empirical physicians, whose tenets he professed to follow, he condemned all curious speculations upon the intimate nature of disease, as incapable of proof, and therefore always useless, and often hurtful; and maintained that the only trustworthy source of opinion in medicine is experience resulting from observations frequently repeated, and experiments cautiously varied; and that no theories worth attention can be framed until the recorded experience of many observers, under many different circumstances, and even through successive ages, shall be embodied into one general system; and he boldly declared his belief that every acute disease might then be cured. An instance, which unfortunately as yet stands alone in support of this rather sanguine expectation, may be taken from the history of small-pox. The observation of its contagious nature led to the general practice of inoculation, and this to the immortal discovery of Jenner, by which a disease but yesterday the scourge of the earth has been almost extinguished. It is remarkable that Sydenham, who first pointed out the important difference between its distinct and confluent forms,—who so materially improved the treatment by changing it from stifling to cooling,—and who studied and has described it with a laborious accuracy hardly paralleled in the history of medicine,—was not aware of this, to us, its most striking characteristic of contagion. A person conversant with such subjects will feel no surprise at this: to the general reader it may be a sufficient explanation, that it lies dormant for ten days; and that as it can only be taken once, and was always prevalent in London, the number of persons susceptible at any given time, and in obvious communication with each other, were comparatively few: so that opportunities were not so likely to arise as might be imagined of tracing its progress in single families or neighbourhoods from one source of contagion.