In this way the chloride of lead is converted into peroxide, while the milk of lime becomes a solution of chlorides of lime and chlorine. This bleaching liquor is decanted from the peroxide of lead, and treated with chloride of lead till it loses its smell of chlorine. The peroxide of lead should bo repeatedly washed with boiling water till it loses all taste, and should then be boiled in a very dilute sodic solution, and again washed till tasteless, when it is in a fit condition for use, and should be kept under water to preserve its impalpable state.
Another process differs from the one we have described only in using permanganate of potash, red lead, or dilute nitric acid to produce the chemical reactions.
3. By the Action of Nitric Acid on Vegetable Substances.—It was proved by Bergmann that oxalic acid might be produced by treating vegetable substances with nitric acid, the beat results being obtained from those which contained no nitrogen, e. g. sugar, starch, and woody fibre. Treacle and coarse sugars have been principally used on account of their low market price, about 116 lb. of oxalic) acid being obtained from 1 cwt. of the former, and 140 lb. from the same quantity of the latter. The operation may be conduoted iu lead-lined wooden vessels or in stoneware jars. The latter are made to hold about two gallons ; they are ranged in rows in water baths, and are heated by steam. The leaden tanks are generally about 8 ft. square and 3 ft. deep, and are provided with a coil of leaden pipe for conducting the steam through their conteuts.
Supposing the manufacture to be conducted in lead-lined tanks, the method of procedure is as follows. About 825 lb. of treacle are run into the cistern, and to this 11 lb. of sulphuric acid are first added, in order to separate out the lime contained in the treacle, when the lime (as sulphate) has settled, the purified molasses is transferred to another tank containing by preference about 15,000 lb. of mother liquors from previous operations, and 900 lb. of nitric acid at 1.200 to 1.270 sp. gr. The contents are well stirred together, and the temperature is increased to about 30° (86° F.) by passing steam through the leaden pipe-coil. This is maintained for twenty-four hours, at the expiration of which the mixture is removed to another vessel, and left for a day or so, in order that the remaining impurities may subside. To the clarified liquor is now added 66 lb. of con
centrated sulphuric acid, as well as 2200 lb. of nitric acid, the latter in quantities of about 3 cwt. at a time and twelve hours apart. The temperature during the first twelve hours of this stage should be kept at about 38° (100° F.) ; during the second twelve hours it is increased to 43° (109° F.); during the third twelve hours it should reach 49° (120° F.), and for the remainder of the operation it may vary from 52° (125° F.) to 51° (129° F.).
About twenty-four hours later the mother liquors are decanted and the crystals drained, dis solved in clean water, and recrystallized. It is said that the use of the mother liquors is essential to procure good results.
Care must be taken that the strength of the nitric acid used shall not exceed the limit mentioned above, or the saccharine matter may be converted either into carbonic or formic acid. The propor tions of the materials used will vary to some extent, according to the nature of the substances used, but when good molasses is employed the amount of nitric acid produced by the action of 320 lb. of sulphuric acid on 278 lb. of nitrate of soda, should be sufficient for the oxidation of 1 cwt. of the treacle, and should yield 100 lb. of marketable oxalic acid.
Some careful experiments by L. Thompson yielded 174 oz. of oxalic acid from 28 oz. of raw sugar but when the mother liquors of previous operations were employed the product was increased to 30 to 321 oz. of the crystallized acid, accompanied by 20 to 24 oz. of carbonic acid.
The chief fault of tbis process lies in the waste of the nitrogen compounds which are disengaged by the oxidation of the saccharine matters, and for the prevention of which many plans have been tried with more or less success. From the fact that these compounds are mixed with carbonic acid, which exercises a remarkable influence in counteracting their affinity for oxygen, one of the most obvious uses to which they might be applied, viz. the manufacture of sulphuric acid, is put out of the question.
One plan of indirectly overcoming this obstacle has been undertaken by Firmin, who passes the gases up a column packed with coke, down which sulphuric acid is made to trickle. The nitro-sulphuric acid thus obtained may be injected into the sulphuric acid chambers in the form of spray, and will thus serve the same purpose as nitrate of soda or nitric acid.