Among other plans proposed we may mention Ecarnot's, in which the gases are mixed with steam and with air from a blast, and the whole then passed over the surface of pumice stone. M'Dougall & Rawson have patented the method of withdrawing the waste gases by an air pump through a series of Woulfe's jars filled with water, and into which air is admitted, while Dale employs the liberated nitrogen vapours for the peroxidation of the protesalts of tin.
4. Manufacture with Alhalies.—So long ago as 1829, it was found by Liebig that a certain quan tity of oxalic acid might be formed by heating neutral organic materials, such as woody fibre, starch, &c., with soda or potash in excess, and about thirty years elapsed before this discovery was applied practically to of the acid.
M. Possoz, who was one of the first to take the matter up, made a series of experiments with various substances to test their relative yield of the acid, the result of which, as published by him, was substantially as follows. By the action of 300 parts of caustic potash en 100 parts of When using starch, the experiment was conducted as follows. The caustic ley was concentrated up to a boiling point of 437° F., and then cooled down to 356° F., upon which the starch was gradually lutroducod In small quantities, the compound being maintained for about four hours at a temperature varying between 392° and 437° F. When the mixture had become white it was heated with water.
When employing other materials, the experiments r found it better to mix them with a caustic ley of sp. gr., and then to evaporate. The woody fibre is thus dissolved ; and should the com pound retain a brown colour after becoming thick, it will be found that it contains a quantity of ulmie acid, but neither oxalic, acetic, nor carbonic acids. But by heating tho mass to between 200° (392° F.) and 225° (437° F.), and maintaining it at that point for some four or five hours, the colour will change to yellow, and ultimately nearly to white, and the three acids named will replace the ulmie acid. The process may be finished more rapidly by increasing the temperature, but it will have the effect of reducing the production of oxalic acid.
When the potash is replaced by caustic soda a different series of reaction ensues. First, the
woody fibre is dissolved and ulmic acid is formed as in the ease of potash, but when the heat is augmented and maintained the acetic, formic) and oxalic acids apparently decompose one another as they form, and hence it is impossible to obtain 10 per cent of the yield rendered by potash even under the most favourable conditions; and from such substances as loather, wool, and silk absolutely no oxalic acid can be derived. There is no difficulty in observing the presence of these three acids throughout the whole process, but at whatever point the operation may be checked, or however much caustic soda be used, the yield of oxalic acid is very small. It would seem as if the production of oxalic acid by the agency of soda is only momentary, probably due to the fact that the latter is much less readily melted than potash and acts more energetically. When, however, the two alkalies aro mixed together in such proportions as to have the same melting point, the product of oxalic acid equals and even excels that yielded by potash alone. The action of the potash appears to be increased by the addition of a certain amount of sodium hydrate, causing a larger quantity of organic matter to be decomposed, and giving a higher produce of oxalic acid with the same weight of alkali.
The results of Possoz's experiments upon the best proportions of the alkalies may be summed up thus :—By the addition of 1 part of hydrate of soda to 3 parts of hydrate of potash, ten per cent. more starch is decomposed than when potash alone is employed, and the yield of oxalic acid is augmented in direct proportion to the amount of starch used ; 1 part of soda hydrate to 2 parts of potash hydrate decomposes -k more starch with proportionally increased yield ; the two alkalies mixed in equal proportions behave similarly to tho potash hydrate alone ; 2 parts of the hydrate of soda to 1 of the potasb, give 10 per cent, less yield than the potash alone; while 3 parts of the soda hydrate to 1 of the potash produce 20 per cent. less oxalic acid, It is evident therefore that hydrate of soda cannot be made to replace potash, but that when mixed with the latter in certain proportions it has a decidedly beneficial influence upon the manufaoture.