What that condition of the system is, which, in forms of fever that are thus mutually convertible, causes one to persist in an uninterrupted series, another to remit, and another, after disappearing for a time, to recur in distinct and regular paroxysms, is wholly unknown. Sydenham, indeed, cuts the knot and removes the difficulty at once. Speaking of the return of the fits in intermittent, he replies to the inquirer into their cause,—“I would fain know why a horse comes to his growth in seven years and a man at twenty-one; or why some plants flower in May and some in June. I am persuaded that the progress of nature is as certain and regular in this case as in any other, and that the matter of a quartan and tertian ague is subject to Nature’s laws and governed by them, as well as any other bodies whatever.” The regularity of nature in the production of disease, no less than in the maintenance of health, cannot be doubted: but the point in question is not clearly one of those ultimate facts, into the reason of which it is wholly vain for the human mind to inquire. Hitherto, however, no one appears to have hazarded even a conjecture as to the cause of this striking difference between these two forms of disease; and pathology, as has just been observed, has afforded no clear light to enable us to determine whether the febrile circle of organs is similarly affected in both. Examinations of fatal cases have been made; but none on that large scale and with that accuracy which alone can render them of any value. I have endeavoured to ascertain the morbid appearances in the spinal cord and the brain, and in the mucous membrane of the respiratory and digestive apparatus, from those who have been long engaged in extensive practice in districts in which ague prevails: but I have been able to obtain no satisfactory answer, excepting that intermittent does not kill! Greatly as the severity of intermittent is without doubt diminished, in the present age, yet we cannot receive such an account without blessing the bark of the seventeenth and the skill of the nineteenth century! A similar want of knowledge exists relative to the condition of the organs in most of the Exanthemata. To supply that want in regard to the various forms of fever that prevail in this metropolis, which, there is good reason to believe, differ but little from the types that appear in other parts of the country, is one of the chief objects for which this work is undertaken.