Mercury, as was already observed, does not always produce a cure from external application. Where this fails, the cure is often effected by internal use. Mercury, from being used internally, comes to be applied to parts otherwise inaccessible, and even to the interior surface of superficial sores, to which, from external application, it cannot penetrate. From this, it is obvious on what principles the internal use of mercury co-operates in the cure of _lues venerea_ with external application; or produces a cure alone where it is not used externally at the same time. Although the cure of _lues venerea_ from mercury may often correspond with the quantity of the evacuation, yet it frequently happens, that, from the early increase of any one particular discharge, the cure is frustrated. This is particularly experienced in those patients in whom the almost immediate effect of mercury is to excite salivation. While this fact is a very strong objection against the supposition that mercury cures by operating as an evacuant, it can readily be accounted for upon the theory here adopted. In such habits, the quantity of mercury accumulated in the circulating fluids can never be very great, as it finds a ready outlet from the system. Besides these phænomena, this theory affords a ready solution for many others also. From hence it is easy to account for the obstinacy of this disease after it has affected the bones. When that has happened, the application of mercury to the part affected, whether it be externally applied, or taken internally, can only be obtained after long continued use. From this also we can learn the reason why the venereal disease will sometimes return, without any new infection, after a seemingly complete cure by means of mercury. The disease will be apparently cured, because the mercury may have totally destroyed the poison at some particular parts; but it will return from the virus still remaining lodged in others, to which, from a course sufficient to destroy it in more accessible parts, the mercury could never penetrate. It appears then, that to the other arguments formerly adduced in favour of this theory, may be added, the ready solution it affords for various phænomena observed with respect to the cure. And, from all the arguments taken together, it may be concluded, that this theory is to be adopted, if not as absolutely certain, at least, as less embarassed with difficulties, and as supported by more probable arguments than any other. ------------------------------------------------------------------------ CHAP. IV. _Of the different Mercurial Preparations employed in Medicine._