attention to the formation of that acid, and much curious and important information has resulted from the experiments of Hermbstadt, Westrumb, Berthollet, Fourcroy, and Vauquelin, &c. but the properties of the acid itself, have been rather neglected. My object in the following pages is not to give a complete history of the properties of oxalic acid, but merely to state the result of a set of experiments, undertaken with the view of ascertaining different particulars respecting it, which I conceived to be of importance. I. Water of Crystallisation. Oxalic acid is usually obtained in transparent prismatic crystals more or less regular; these crystals contain a portion of water, for when moderately heated they effloresce, and lose a part of their weight, which they afterwards recover when left exposed in a moist place. When cautiously heated on a sand-bath they fall to powder, and loose about a third of their weight. But as the acid is itself volatile, it is not probable that the whole of this loss is water. To ascertain the quantity of water contained in these crystals I had recourse to the following method. 1. Seventy grains of crystallised oxalic acid were dissolved in 600 grains of water, constituting a solution which weighed 670 grains. Fifty grains of pure carbonate of lime, in the state of calcareous spar, were dissolved in muriatic acid; this solution was evaporated to dryness to get rid of the excess of acid, and the residue redissolved in water. Into this muriate of lime the solution of oxalic acid was dropped by little and little as long as any precipi tate tate fell, and the oxalate of lime thus formed was separated by the filter. Pure oxalic acid is not capable of precipitating the whole lime from solution of muriate of lime, the muriatic acid evolved being always sufficient to retain the last portions in solution. It was necessary to get rid of this excess of acid; the method which appeared the least exceptionable was to saturate the muriatic acid with ammonia; accordingly, when the oxalic acid ceased to occasion any farther precipitate, I cautiously added pure ammonia, till the liquid ceased to produce any effect upon vegetable blues. A copious additional precipitate of oxalate of lime was thus obtained. Oxalic acid was now added again as long as it rendered the liquid muddy. By thus alternately having recourse to the acid solution, and to ammonia, and by adding both with great caution to avoid any excess, I succeeded in separating the whole of the lime without using any sensible excess of oxalic acid. 558 grains of the acid solution were employed, a quantity which is equivalent to 58.3 grains of the crystallised acid. 2. The oxalate of lime, after being well washed and drained, and exposed for a week to the open air, at a temperature of about 60°, weighed 76 grains; but upon being left on the sand bath for some hours in a temperature between 200° and 300°, its weight was reduced to 72 grains. 3. These 72 grains of dry oxalate of lime were put into an open platinum crucible, and gradually heated to redness. By these means they were reduced to 49.5 grains, which proved to be carbonate of lime. The crucible was now exposed to a violent heat in a forge. Nothing remained but a quantity of pure lime weighing 27 grains. 4. From this experiment we learn, that 72 grains of dry oxalate of lime contain 27 grains of lime. Of consequence, the oxalic acid in this compound must be 45 grains. But the weight of crystallised oxalic acid actually used was 58.3 grains, a quantity which exceeds the whole acid in the oxalate by 13.3 grains. These 13.3 grains are the amount of the water of crystallisation, which either did not unite with the salt, or was driven off by the subsequent exposure to heat. Hence crystallised oxalic acid is composed of So that the crystals of oxalic acid contain very nearly the fourth part of their weight of water *. II. Alka * Vauquelin in a late dissertation on cinchona, marked with that profound skill which characterizes all the productions of this illustrious chemist, has mentioned incidentally, that the crystals of oxalic acid contain about half their weight of water. He dissolved 100 parts of cinchonate of lime in water, and precipitated by means of oxalic acid; 22 parts of crytallised oxalic acid were necessary; and the oxalate of lime formed weighed 27 grains. From this experiment he draws the conclusion which I have stated, (Sec Ann. de Chim. lix. 164.) But this ingenious chemist does not seem to have been aware of the real composition of oxalate of lime; 27 grains of that salt are composed very nearly of 10 grains of lime and 17 grains of acid. But the weight of the crystals used by Vauquelin was 22; the difference, 5, is obviously the water of crystallisation in 22 grains of the crystals. But if 22 grains contain 5 of water, it is obvious, that II. Alkaline and Earthy Oxalates. 1. The preceding experiment gives us likewise the composition of oxalate of lime. This salt, when merely dried in the open air, still retains a portion of water which may be driven off by artificial heat. It is necessary to know that it parts with this water with considerable difficulty, so that a long exposure on the sand or steam bath, is necessary to get it thoroughly dry. It afterwards imbibes a little water if it be left in a moist place. Well dried oxalate we have seen is a compound Though the oxalate of lime dried spontaneously can scarcely be considered as always in the same state, yet as the difference in the portion of water which it retains is not great, provided it be dried slowly in the temperature of 60o, and in a dry place, it may be worth while to state its composition. It is as follows : Acid 45 or per cent. 59.2 acid. Base 27........................ 35.5 base. When rapidly dried, as by pressing it between the folds of filtering paper, it is apt to concrete into hard lumps, which retain more moisture. In this state I have sometimes seen it retain 10 per cent. of water after it appeared dry. 100 contain very nearly 23. So that his experiment in reality coincides with mine. Bergmann spar His method was to dissolve a determinate quantity of calcareous in nitric acid, and then to precipitate the lime by oxalic acid. 100 parts of calcareous spar thus dissolved, require, according to him, 82 parts of crystallised acid to precipitate them. But there must have been some mistake in this experiment; for, according to my trials (provided the nitric acid be carefully neutralised by ammonia as it is evolved), no less than 117 grains of oxalic acid would have been required, and at least 145 grains of oxalate of lime would have been obtained instead of the 119, which was the result of Bergmann's experiment. It is obvious that Bergmann did not precipitate all the lime. He added oxalic acid till it ceased to produce any effect on the solution from the great excess of nitric acid evolved; and then took it for granted that all the lime was separated. But had he added ammonia, he would have got an additional quantity of oxalate of lime, and the precipitation would have recommenced upon adding more oxalic acid. This explanation accounts in a satisfactory manner for the difference between Bergmann's statement of the composition of oxalate of lime, and mine. 2. Though the preceding experiment was made with care, yet as some of the most important of the following * Opusc. L. 262. observations |