Now, though in such cases the action of a medicine seems to be continued from the part where it is situated to a distant point, there are certainly no authenticated examples of such an action by a remedy in the blood. Yet it would not be reasonable to deny the bare possibility of such a thing. But such instances are at all events rare, and, being cases of the manifestation of their vital properties by nerve and muscle, rather than of the proper action of a medicine, must not be considered to invalidate the above proposition, which, as a general rule, is of considerable importance. PROP. VI.--_That while in the blood, the medicine may undergo change, which in some cases may, in others may not, affect its influence. That these changes may be--_ _a._ _Of Combination._ _b._ _Of Reconstruction._ _c._ _Of Decomposition._ Before advancing to the consideration of the modes in which medicines operate in the cure of disease, it is of importance to mention that some of them are liable to undergo changes in the animal organism, because in particular cases these changes may materially affect their action, and in all cases they have some bearing upon it. With a view to this point I have divided these changes, somewhat arbitrarily, into three kinds,--of which the first does not hinder the effect of a medicine--the second may alter it slightly--and the third entirely changes or neutralizes it. Some of these change have to be considered more at length in the progress of the Essay, so that here I will only give an outline of them. _Changes of Combination._--The chief, and indeed almost the only way of detecting changes in medicines, is by taking notice of their effect upon the composition of the secretions. Now in the blood we have a slight excess of alkali; in the urine, an excess of acid. Acids and alkalies are often given as medicines; and as it is not right that there should be much excess of either in the blood, both are generally soon neutralized in the system, and reduced to salts. This may sometimes occur before absorption, but perhaps more often after it. Now, it may seem strange for me to say that this neutralization does not destroy their influence. But it evidently does not; for in being neutralized they diminish in the blood, and in the system generally, the quantity of basic or of acid matter, and thus tend to alter the reaction of the secretions. Though an acid may combine in the blood with Soda, or with salts of Soda, yet by so doing it causes an excess of some other acid, probably an animal acid, which, being set free, acts on the secretion of urine much in the same way that the first acid would have done. It is easier to render the urine alkaline than to make it acid, for two reasons; first, as I shall show hereafter, an acid may pass out through other glands besides the kidneys; and secondly, an alkali is not so easily neutralized, either before or after absorption, the blood being already more or less alkaline.