------------------------------------------------------------------------ CHAP. II. _Concerning the Opinion, that Mercury cures the Lues Venerea by the evacuation it produces._ THERE are few medicines with regard to the operation of which all practitioners are agreed. It is, however, by no means incumbent on him who means to establish the truth of any one opinion to overturn every hypothesis advanced on the same subject. But, as the arguments urged in favour of any hypothesis will have much less weight when another is previously believed to be true, and as the regulation of practice is, in every case, to a certain extent, founded upon theory, no inconsiderable advantage may be derived from overturning an ill-founded opinion, especially when it is generally received. The theory of the action of mercury, as well as of other substances, has afforded room for a diversity of opinions. For the reasons mentioned above, then, although it is by no means intended, that every opinion, with regard to the operation of that medicine, should here be considered; yet, as it is a very prevailing opinion, that the good effects obtained from mercury in the cure of _lues venerea_, depend upon its action as an evacuant; and, as a variety of seemingly strong arguments have been adduced in favour of that hypothesis, it will be necessary to examine how far they are sufficient to establish its truth. In favour of the opinion, that mercury cures _lues venerea_ by acting as an evacuant, the following arguments have been employed. It is alledged, that the good effects obtained from mercury in the cure of this disease, are in proportion to the evacuation which it produces; that the cure produced by mercury depends more upon the stimulant power of the preparation which is employed, than upon the quantity taken; and that the same good effects are obtained from other evacuants as from mercury; particularly, that the venereal disease is cured in a similar manner from the employment of guaiac. The arguments here enumerated, if not the only ones upon which this opinion is founded, are at least those which are principally employed. To examine, then, how far these are well founded, will be sufficient. The first argument here adduced is, that the good effects of mercury are proportioned to the evacuation which it produces. This assertion, if allowed to be true, might, at first sight, appear to be a very strong argument in favour of the theory here adopted. But it is strong in appearance only; for, although it should be admitted, it in fact proves nothing.