736

ELEMENTS OF MATERIA MEDICA.

and with solutions of the vegetable alkalies, white precipitates ( tannates ), slight y .luble in water, but very soluble in acetic acid. The mineral acids also cause P* e leS .tates with concentrated solutions of tannic acid, as do the alkalies and their carbon | eGelatinous alumina rapidly absorbs tannic acid from its solution, and forms an msocompound with it. . -^t

Tannic acid is composed of CIS H 8 O 12 ; consequently its equivalent or atomic w ris 212. . s it

Tannic acid is a very powerful astringent. Given todogstothe extent of 12 g ra , ecaused constipation. One of the animals being killed, the intestinal mucous inenI j . r jjaSwas found dry, and the fiscal matter hard, and collected in the colon. In doses of two Land a hal fit produced constipation in the human subject (Cavarra, Z/ond. Med. Gaz . v0 jj c j.p. 171). To the presence of this acid the vegetable astringents principally owe their 111 u s,nal activity (vide p. 81). It has been employed in hemorrhages (from the lungs, u ^and rectum), and in profuse mucous discharges (diarrhoea,pulmonary catarrh, leucorr ^and gonorrhoea). It may be administered in doses of three grains, in the form 0 "or solution. It presents but few advantages over the astringent extracts. j]jc

2. Gallic Acid (Acidurn Gallicum .)—Though we obtain nearly 20 per cent, of S - 0 .acid from nutgalls, these excrescences contain very little of it, our produce being P ^cipally the result of the decomposition of the tannic acid. Nay, Pelouze thinkseven the small quantity of gallic acid which does exist in nutgalls, is formed n.Y gaction of the air on the tannic acid during or subsequent to the process of drying 1bodies. . fte

The conversion of tannic into gallic acid is effected by the agency of the air, ^ tf ooxygen of which is absorbed, while an equal volume of carbonic acid is evolved.atoms of tannic acid absorb thirty atoms of oxygen from the air, and yield 3 agallic acid, 15 atoms carbonic acid, and 9 atoms water.

Carb. Hyd. Oxyg.atms. atras. atms.2 atoms Tannic acid consist of 36 18 24

30 atoms Oxygen of the air .. 0 0 30

Total .

36

18 54

carb - Hyi : ?

atms. atms- ^

3 atoms Gallic acid consist of .. 21 *j 3<>

15 atoms Carbonic acid. 15 9

9 atoms Water. ^

is 54

Total. 36 18

When the air is excluded no gallic acid is formed. _J » t rp“!

Pure gallic acid is a colourless, crystallizable solid, with an acidulous an ^ rC(jT ntaste. It produces a deep blue colour with the sesquisalts of iron, in which - ta ti(*gstance it agrees with tannic acid ; but it differs from the latter acid in not prect) a ci”’gelatin or the vegetable alkaline salts. To detect gallic acid mixed with , . anI1 () fsk> 1 !the latter is to be previously removed from its solution by immersing in it a P] eC j e tec teadepilated by lime. The tannic acid is absorbed. The gallic acid may then be ^by the salts of iron. . . py'lif 8

Gallic acid consists of C? H 3 O* ; hence its equivalent or atomic weight isheated to 410° or 420° F., it gives out carbonic acid, and is resolved into ]>H -qacid (C6 H 3 O 3 ). If the heat be raised to 480° F., both water and carbonic aevolved, and metagallic acid (C 12 H 3 O 3 ) is produced.

The effects and uses of gallic acid have been before noticed (p. 81). itne<J *i

3. Ellagic acid (Acidum Ellagicum). — Discovered by Braconnot, who n ^jpe®ellagic acid, from the French word for galls ( galle ) spelt backwards. It 1S . ^ a 1 ' 11from galls in the process for making gallic acid, and hence is probably a pt° l | o j r o 8not an educt. It is a white, insipid powder, which becomes of a blood-red cothe addition of nitric acid. It consists of C7 H 2 (H ; hence its equivalent 0weight is 76.

Chemical Characteristics. —Infusion of nutgalls reddens ;t j tpaper, forms an inky compound ( tanno-gallate of iron) on the a 0 jof a sesquisalt of iron, and a yellowish white precipitate put 8 ’

gelatin) with a solution of gelatin. If a piece of skin, depilated )be immersed in the infusion, and agitated with it from time to tnthe tannic acid is absorbed, the filtered liquid striking a blu ■ - ta tc(gallate of iron) with the sesquisalts of iron, but gives no p re

illS

iofl