- It is undoubtedly true, that in some orders and classes of plants, a strong similarity in action and properties pervades each individual of the class; but at the same time, there are so many exceptions to be found, that it is impossible to place any confidence in it as a means of discriminating beforehand, whether or no, a certain individual, hitherto untried, possesses the same properties as others of the family to which it belongs. A few examples will suffice. The order Solanaeeae comprises Belladonna, Stramonium, and Tobacco, all powerful sedatives, and Capsicum annuum, a violent acrid stimulant; the order Liliaceae contains the Aloe, a drastic purgative; Scilla maritima, the Squill, an expectorant and emetic; and the common Asparagus, an innocuous vegetable. In the Cin-chonaceae we find Cinchona, a tonic; and Ipecacuanha, an emetic; in the UmbelliferAe, Conium, Hemlock, a sedative; the common Cumin, a stimulant and carminative; and the Daucus Carota, or Carrot, an innocent, wholesome article of food; and lastly, in the Cucurbitaceae we meet with the Momordica Elaterium, a powerful hydragogue, and the Cucumis sativa, the common Cucumber, and Cucumis Melo, the Melon, both innocent articles of diet. It should be further observed, that many articles, possessing very similar medical properties, belong to widely different orders. It is only necessary to mention Digitalis and Tobacco, both powerful depressants, and in some points resembling each other in therapeutic action; the former belongs to the order Scrophulariaceae, the latter to the Solanaceae.

* Animal Chemistry, 2nd Ed. p. 170.

2. Similarity in chemical composition has been suggested, as indicating a similarity in therapeutic action, but there is less reliance to be placed on this test than even on that of the botanical characters; although in some instances, as, for example, the strong mineral acids, and the fixed alkalies, where there is a close chemical relation, very analogous effects on the system are produced. But these should be regarded rather as the exceptions to than as the supporters of the rule. The following is one out of many examples of chemical affinity and dissimilar therapeutic action: -

Citric Acid, a mild refrigerant,

3 HO, C12 H5 O11

+ HO.

Gallic Acid, a powerful astringent

3 HO, C14 H3 O7

+ 2HO.

If we turn to some of the alkaloids, we find the fallacy of this test strikingly illustrated. The chemical formulAe of the six following alkaloids may be quoted as examples: -

Quinia .

C40 H24 N2 O4.

Strychnia .

C42 H22 N2 O4.

Beberia .

C38 H21 N O6.

Morphia .

C34 H19 NO6.

Emetina .

C37 H27 NO10.

Aconitia .

C60 H47 NO14.

Much stress has been laid on the fact that Nitrogen enters into the composition of almost every substance which powerfully affects the nervous system. However true this may be, there are numerous substances containing Nitrogen which exercise no such influence.

3. The sensible properties of medical substances have been suggested as a test of their therapeutic action; but no confidence whatever is to be placed on this test, as may be shown by one example. Sulphate of Magnesia, Sulphate of Zinc, and the crystals of Oxalic Acid, so closely resemble one another in external characters, that they have often been substituted by mistake, the one for the other; yet how different are their effects - the first is a purgative, the second an emetic, and the third a virulent poison.