GAL
r 109 )
G L A
In our former article we described Scheele ’soriginal process for preparing this acid, andalso Deyeux’s method by sublimation. Mr.Richter has given a different process, by whichthis acid is prepared in very considerable purity.It is the following. 1 Make a cold infusion offinely powdered gall-nuts in water, frequentlystirring it, then strain the whole through aeloth, and press the residue strongly to get outthe whole infusion. Evaporate the liquor todryness with a very gentle heat, which willleave a dark brown brittle mass. Reduce thisto a fine powder and digest it with very highlyrectified alcohol, added twice successively. Thofirst tincture thus prepared will have a weakstraw-colour ; the second will be nearly colour-less. The undissolved part is tannin almostpure. Mix both the spirituous tinctures, anddistill it in a retort till only an eighth part isleft. Add water to the residue in the retortand warm it, and then reduce it by evaporation,and small white crystals will separate, as longas any mother liquor remains. These crystalsare the Gallic Acid , One pound of galls willgive about half an ounce of these crystals.
This acid certainly contains a smaller ad-mixture of tannin than that prepared in Scheelc’smethod ; for tannin is scarcely at all soluble inalcohol, whereas gallic acid is highly so, butthe alcohol here employed should be very highlyrectified.. This acid when added to a perfectlyclear and neutral solution ofsulphatof iron doesnot produce any change till the iron has ab-sorbed an additional portion of oxygen from the-atmosphere. In this respect it differs fromScheele ’s acid, or from the simple infusion ofgalls, which immediately gives a purple hueto the neutral sulphat of iron. Richter there-fore supposes, with some probability, that thetannin in this case unites with the sulphuricacid, and detaches a portion of oxyd of iron tounite with the gallic acid.
Bouillon La Grange has given a valuablecomparative view of the shades of differencein these acids, with some remarks on theintimate nature of gallic acid. b Berthollet having proposed the oxyd of tin as a methodof freeing the common brown gallic acid fromits adhering tannin or extract, La Grange has-examined this process, and finds that by eva-porating a solution of this brown acid in contactwith a certain quantity of the oxyd of tin, andthen again diluting with water, the acid stillgave a precipitate, with animal jelly, though
a Ann. Chim. tom. 52.
slightly, and the solution when crystallizedfurnished a gallic acid of greater purity thanbefore. But on repeating the process with afresh portion of the oxyd of tin, the solutionwas now so entirely decomposed as to be clearand colourless, to give no precipitate eitherw-ith sulphat of iron or with jelly, and to yieldno crystallized acid on evaporation.
The red oxyd of mercury when long boiledwith the gallic acid, finally decomposes boththe pure acid as well as the tannin.
Charcoal recently calcined, when boiled withgallic acid, deprives it of its peculiar taste;gives it a green- colour, and takes away itsproperty of precipitating jelly; and a greaterproportion of charcoal destroys the' acid alto-gether.
Ail these experiments shew the impossibilityof obtaining gallic acid free from the tanninexcept by sublimation, but it remains to beshewn whether the sublimed acid is identicalwith the mere acid part of the crystallizedacid. M. La Grange has contrasted the dif-ferences between these two acids. The sub-limed acid, he observes, has less acidity, it is"decomposed in the air, it has no action onbarytes, or carbonat of ammonia, or on muriatof tin; the precipitate obtained by nitrat ofmercury is blackish instead of being yellow,anti that by acetite of lend is yellow and scanty,instead of being white: oxymuriatic acid renders,it brown, whereas it does not change the-colour of the crystallized acid. The sublimed;acid does not precipitate animal jelly.
In continuing these experiments both on the'acid and on the gall nut itself, the author'uniformly obtained a quantity of acetous acidlby the distillation of all these varieties of gallicacid, whence he is led to the conclusion thatthe acetous acid is the real acid basis of thegallic acid, as well of most other supposed-individual vegetable acids, and that its peculiarcharacters are masked by volatile or fixed em-pyreumatic oils, by tannin, by extract, or byother vegetable principles with which it iscapable of entering into intimate combination.
glauberite.
The colour of this mineral is wine yellowof greater or less intensity: it occurs only-crystallized, in the form of an oblique prismwith rhombic bases. The faces of the base-are smooth and brilliant, those of the sides are-striated parallel to the edges of the base. Ifpresents natural joints in two directions, namely,parallel to the base (and these are the-easiest,b Ann. Cilia 1 ., tom. f)0.