There is commonly a tendency to describe all medicines under two heads, as either causing or diminishing vital activity. Dr. Murray indeed confounds these two effects, and adopts an idea on this subject which was originated by Mr. Brown. I shall now represent in an abridged form the classifications adopted severally by Dr. Young, Dr. Duncan, and Dr. Murray. DR. YOUNG. 1. CHEMICAL AGENTS. Caustics, etc. 2. VITAL AGENTS. A. _Supporting strength._ Nutrients. B. _Causing action._ (Partial and transitory.) Stimulants, Irritants, Astringents, Alteratives, Evacuants. (Permanent.) Tonics. C. _Diminishing action or sensation._ (Primarily.) Narcotics, Nauseants, Sedatives, Diaphoretics. (Secondarily.) Exhaurients. 3. INSENSIBLE AGENTS. Specifics. In the names of these three classes some reference is made to the _modus operandi_ of medicines, but the distinctions thus attempted to be drawn are of the slenderest possible description. Among _chemical_ agents are included some that are applied externally, and act then on the tissues in obedience to known chemical laws. But how can we affirm that some medicines passing into the stomach may not operate chemically? The term _vital_ signifies little; and the word _insensible_, applied to the third class, is a confession of absolute ignorance. It does not attach to a medicine any distinctive character to say that we know nothing of its operation. Pursuing the subdivision further, we find that the distinctions are not well maintained. Tonics support strength as well as cause action; and it can hardly be said that the action of an Alterative, such as mercury, is partial and transitory. It seems unwise to have made a separate class of Specifics. They are especially associated with Alteratives. Mercury, Iodine, and others, would fall under both groups. Of Evacuants it may be observed, that they are also Exhaurient, and thus included in two opposed classes; that their action is not always partial and transitory, as, for example, Purgatives may permanently remedy constipation; and that it seems wrong to have separated Diaphoretics from them.