PHYSIOLOGICAL CLASSES OF MEDICINES.

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dilute 6tate, allay thirst, promote the appetite and digestive process, andaugment the secretion of urine. By continued use, they reduce the heatof the body, diminish the fulness and quickness, but increase the firmnessof the pulse, check the cutaneous and pulmonary exhalation and secre-tion, and heighten the general tonicity of all the fibres and organictissues. If their employment be continued for too long a period, thedigestive functions become much disturbed, chronic inflammation of themucous lining of the alimentary canal is set up, accompanied with wastingand disorder of the whole system. They are employed as cooling andtemperant means in fevers, especially of the hectic kind, and likewise astonics. They are useful adjuncts to some of the bitter infusions.

f. The sixth group includes the metallic tonics, and consists princi-pally of the preparations of iron. These combine tonic and stimulantproperties, and will be noticed hereafter.

Active principles. —The substances contained in the vegetable tonics,and on which their activity depends, are alkaloids, crystalline sub-stances analogous to the alkaloids, tannic and gallic acids, and ex-tractive.

1. Tonic alkaloids. —These are quinia, cinchonia, and aricina: theirproperties will be examined hereafter.

2. The crystalline substances analogous to vegetable alkalies found inthe vegetable tonics, and which possess medicinal activity, are salicine,gentianine, quassine, &c. These are too imperfectly known to permitany general account to be given of them.

3. Tannic acid (acidum tannicum). —As this substance is employed inmedicine, it will be described in a subsequent part of this work. It willbe sufficient, therefore, here to state that its presence in the astringenttonics is shown by the whitish, or yellowish white precipitate, ( tanno-gelatin) which infusions of these substances form with a solution ofisinglass, and by the blue or green precipitate (pertannate of iron) whichthey give on the addition of a perferruginous salt. The followingastringents produce a bluish black precipitate with the persalts ofiron: bistort, oak-bark, nutgalls, logwood, pomegranate-rind, red roseleaves, and uva ursi. The persalts of iron give rise to a green preci-pitate with the barks of cinchona, willow, elm, and cinnamom, withcatechu, kino, tormentilla, rliatany, and wormwood. Tannic acidusually causes precipitates ( tannates) with the vegetable alkalies.

4. Gallic acid (acidum gallicum). —The properties of this acid are verysimilar to those of tannic acid. From this circumstance, as well as fromthe fact that gallic acid is easily produced by the action of air on tannicacid, it is difficult to prove whether certain vegetable substances containboth these acids, or only tannic acid. Gallic acid agrees with tannicacid in producing a deep blue colour with the persalts of iron, but itdoes not precipitate gelatine or the vegetable alkalies. Though obtainedfrom several vegetables, yet it probably either does not exist in many ofthem,oris present in very small quantities only: it is to be regarded, in mostcases, as a product rather than an educt. Thus, though nutgalls yieldone-fifth of their weight of gallic acid, Pelouze thinks that, originally,they contain none of it, but that what is procured is obtained by theaction of atmospheric air on the tannic acid. Taken internally, in smalldoses, gallic acid causes no inconvenience. It was once given in the doseof from 15 to 30 grains, against the Tania Solium, but without any

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