It has been said that mercury does no service unless it purges or salivates. I am disposed to believe that it may act as a counter stimulus to that of the miasmata of the yellow fever, and thus be useful without producing any evacuation from the bowels or mouth. It more certainly acts in this way, provided blood-letting has preceded its exhibition. I have supposed the stimulus from the remote cause of the yellow fever to be equal in force to five, and that of mercury to three. To enable the mercury to produce its action upon the system, it is necessary to reduce the febrile action, by bleeding, to two and a half, or below it, so that the stimulus of the mercury shall transcend it. The safety of mercury, when introduced into the system, has three advantages as a stimulus over that of the matter which produces the fever. 1. It excites an action in the system preternatural only in _force_. It does not derange the _natural_ order of actions. 2. It determines the actions chiefly to external parts of the body. And, 3. It fixes them, when it affects the mouth and throat, upon parts which are capable of bearing great inflammation and effusion without any danger to life. The stimulus which produces the yellow fever acts in ways the reverse of those which have been mentioned. It produces violent _irregular_ or _wrong_ actions. It determines them to internal parts of the body, and it fixes them upon viscera which bear, with difficulty and danger, the usual effects of disease. A late French writer, Dr. Fabre, ascribed to diseases a centrifugal, and a centripetal direction. From what has been said it would seem, the former belongs to mercury, and the latter to the yellow fever. Considering the great prejudices against blood-letting, I have wished to combat this fever with mercury alone. But, for reasons formerly given, I have been afraid to trust to it without the assistance of the lancet. The character of the fever, moreover, like that which the poet has ascribed to Achilles, is of "so swift, irritable, inexorable, and cruel" a nature, that it would be unsafe to rely exclusively upon a medicine which is not only of less efficacy than bleeding, but often slow and uncertain in its operation, _more especially_ upon the throat and mouth. Let not the reader be offended at my attempts to reason. I am aware of the evils which the weak and perverted exercise of this power of the mind has introduced into medicine. But let us act with the same consistency upon this subject that we do in other things. We do not consign a child to its cradle for life, because it falls in its first unsuccessful efforts to use its legs. In like manner we must not abandon reason, because, in our first efforts to use it, we have been deceived. A single just principle in our science will lead to more truth, in one year, than whole volumes of uncombined facts will do in a century.