OXALATES

are saline compounds of the bases with OXALIC ACID ( Acide oxalique , Fr.; Sauerkleesaüre , Germ.); which is the object of a considerable chemical manufacture. It is usually prepared upon the small scale by digesting four parts of nitric acid of specific gravity 1·4, upon one part of sugar, in a gla retort; but on the large scale, in a series of salt-glazed stoneware pipkins, two-thirds filled, and set in a water bath. The addition of a little sulphuric acid has been found to increase the product. 15 pounds of sugar yield fully 17 pounds of the crystalline acid. This acid exists in the juice of wood sorrel, the oxalis ace to sell a , in the state of a bi-oxalate; from which the salt is extracted as an object of commerce in Switzerland, and sold under the name of salt of sorrel, or sometimes, most incorrectly, under that of salt of lemons. Some prefer to make oxalic acid by acting upon 4 parts of sugar, with 24 parts of nitric acid, of specific gravity 1·220, heating the solution in a retort till the acid begins to decompose, and keeping it at this temperature as long as nitrous gas is disengaged. The sugar loses a portion of its carbon, which combining with the oxygen of the nitric acid, becomes carbonic acid, and escapes along with the deutoxide of nitrogen. The remaining carbon and hydrogen of the sugar being oxidized at the expense of the nitric acid, generate a mixture of two acids, the oxalic and the malic. Whenever gas ceases to i ue, the retort must be removed from the source of heat, and set aside to cool; the oxalic acid crystallizes, but the malic remains di olved. After draining these crystals upon a filter funnel, if the brownish liquid be further evaporated, it will furnish another crop of them. The residuary mother water is generally regarded as malic acid, but it also contains both oxalic and nitric acids; and if heated with 6 parts of the latter acid, it will yield a good deal more oxalic acid at the expense of the malic. The brown crystals now formed being, however, penetrated with nitric, as well as malic acid, must be allowed to dry and effloresce in warm dry air, whereby the nitric acid will be got rid of without injury to the oxalic. A second crystallization and efflorescence will entirely di ipate the remainder of the nitric acid, so as to afford pure oxalic acid at the third crystallization. Sugar affords, with nitric acid, a purer oxalic acid, but in smaller quantity, than saw-dust, glue, silk, hairs, and several other animal and vegetable substances. Oxalic acid occurs in aggregated prisms when it crystallizes rapidly, but in tables of greater or le thickne when slowly formed. They lose their water of crystallization in the open air, fall into powder, and weigh 0·28 le than before; but still retain 0·14 parts of water, which the acid does not part with except in favour of another oxide, as when it is combined with oxide of lead. The effloresced acid contains 20 per cent. of water, according to Berzelius. By my analysis, the crystals consist of three prime equivalents, of water = 27, combined, with one of dry oxalic acid = 36; or in 100 parts, of 42·86 of water with 57·14 of acid. The acid itself consists of 2 atoms of carbon = 12, + 3 of oxygen = 24; of which the sum is, as above stated, 36. This acid has a sharp sour taste, and sets the teeth on edge; half a pint of water, containing only 1 gr. of acid, very sensibly reddens litmus paper. Nine parts of water di olve one part of the crystals at 60° F. and form a solution, of spec. grav. 1·045, which when swallowed acts as a deadly poison. Alcohol also di olves this acid. It differs from all the other acid products of the vegetable kingdom, in containing no hydrogen, as I demonstrated (in my paper upon the ultimate analysis of organic bodies, published in the Phil. Trans. for 1822), by its giving out no muriatic acid gas, when heated in a gla tube with calomel or corrosive sublimate. Oxalic acid is employed chiefly for certain styles of discharge in calico-printing , (which see), and for whitening the leather of boot-tops. Oxalate of ammonia is an excellent reagent for detecting lime and its salts in any solution. The acid itself, or the bi-oxalate of potash, is often used for removing ink or iron-mould stains from linen. A convenient plan of testing the value of peroxide of manganese for bleachers, ., originally proposed by Berthier, has been since simplified by Dr. Thomson, as follows. In a poised Florence flask weigh 600 grains of water, and 75 grains of crystallized oxalic acid; add 50 grains of the manganese, and as quickly as po ibly afterwards from 150 to 200 grains of concentrated sulphuric acid. Cover the mouth of the flask with paper, and leave it at rest for 24 hours. The lo of weight it has now suffered, corresponds exactly to the weight of peroxide of manganese present; because the quantity of carbonic acid producible by the reaction of the oxalic acid with the peroxide, is precisely equal to the weight of the peroxide, as the doctrine of chemical equivalents shows.