the application of artificial cold. Its specific gravity at the temperature of 68° is 1·1168,* while that of the most concentrated acetic acid does not exceed 1.08, and it contains 19.66 per cent. of water. When mixed with its own weight of pure water its specific gravity becomes 1:06. And if we add twice as much water it becomes 1·0296. Class I. Div. II. Dobereiner has ascertained that when mixed with anhydrous sulphuric acid it undergoes decomposition, because it cannot subsist without water or a base. It is converted into carbonic Composition. oxide gas and water. From the proportion of carbonic oxide gas evolved, he concluded that it consists of so that it differs from oxalic acid simply by containing an atom of hydrogen, while oxalic acid contains only 3 atoms of oxygen and 2 atoms of carbon. This corresponds very well with the analysis of Berzelius, who found its constituents almost coinciding with the theoretical numbers. The analysis of Dr. Gobel agrees also with this very nearly. There cannot be the least doubt then, that the atomic constitution of this acid is as has been stated, and that its atomic weight is 4.625. Formic acid has a considerable resemblance to acetic; but they differ in some particulars, particularly when combined with bases. Very dilute formic acid is said to undergo spontaneous decomposition like vinegar. But I have preserved for several years, without any alteration, formic acid prepared by Dobereiner's process, and in a rather dilute state, though much stronger than vinegar. * Schweigger's Jour. iv. 14. Schweigger's Jahrbuch, ii. 344. † Annals of Philosophy, ix. 107. Chap. I. History. SECTION III.-OF MELLITIC ACID. There is a mineral of a honey-yellow colour which is found in small solitary crystals among the layers of wood-coal at Arten in Thuringia. At first sight it has some resemblance to amber; but Werner recognised it as a peculiar substance about the year 1790, and gave it the name of honigstein (honey-stone), which foreign mineralogists converted into mellite. This mineral is very rare. Hitherto indeed it has been found only in Thuringia and in Switzerland.* Mellite has usually a honey-yellow colour, but sometimes a straw yellow. It is always crystallized in octahedrons, but they are rarely entire; sometimes indeed almost the whole of one of the pyramids is wanting. Their surface is generally smooth and brilliant, and interiorly they have a glassy lustre. They are semitransparent, brittle, soft, and easily reduced to powder. When pounded, they assume a yellowish-gray colour. Their specific gravity is about 1·550.† Mineralogists soon discovered that mellite is partly combustible; but they did not agree about its component parts. Lampadius and Abich§ undertook its chemical analysis about the same time. But the results of their analyses differed so much from each other, that little confidence could be placed in either. Besides, it was evident from the way in which their experiments were conducted, that the original component parts of mellite had been altered by fire. Klaproth analyzed it in 1799, and ascertained it to be a compound of alumina and a peculiar acid, to which he gave the name of mellitic.|| And this analysis was soon after confirmed by Mr. Vauquelin. In the year 1826 M. Wöhler subjected this acid to a new examination, and has added several interesting facts to those which had been previously determined.** Hitherto mellitic acid has been found only in the mellite. It may be procured from that mineral by the following process: reduce the mellite to powder, and boil it in about 72 times its weight of water. The acid combines with the water, and the alumina separates in flakes. By filtering the solution, and evaporating sufficiently, the mellitic acid is obtained in the * Brochant's Mineralogy, ii. 75. ** Poggendorf's Annalen, vii. 325. Klaproth's Beitrage, iii. 115. $ Ibid. p. 16. Ann de Chim. xxxvi. 203. state of crystals. This was Klaproth's process. But Wöhler on repeating it found that the mellitic acid was not free from alumina. He employed the following process for procuring this acid in a state of purity: Mellite reduced to a fine powder was digested in a solution of carbonate of ammonia, and the liquid, after it had taken up all the mellitic acid, was boiled till all the excess of ammonia was driven off. It was then filtered and evaporated till the mellate of ammonia was obtained in crystals. The pure crystals were dissolved in water and precipitated by acetate of lead. The precipitate being edulcorated was mixed with water, through which a current of sulphuretted hydrogen gas was passed to throw down the lead. The mellitic acid dissolves in the water, and is obtained in a state of purity by filtration. The liquid obtained in this way being concentrated to the consistence of a syrup did not crystallize, but by continuing the evaporation the acid remained behind in the state of a white powder. Being dissolved in alcohol and left to spontaneous evaporation, the mellitic acid crystallized in very fine needles arranged in the form of stars. Class I Div. II. Mellitic acid has a very sour taste, is not altered by exposure Properties, to the air, is very soluble in water and alcohol, and bears a pretty high temperature before it is charred. When sublimed in a glass tube a crystalline fusible matter is obtained, which possesses peculiar properties, and has obtained the name of pyromellitic acid. During this sublimation no smell of empyreumatic oil is observed, as usually happens when organic bodies are exposed to a temperature sufficiently high to produce volatilization. Sulphuric acid, though in a concentrated state, produces no alteration on mellitic acid. It dissolves the acid by a boiling heat, but no alteration is produced in its colour, and the sulphuric acid may be distilled off leaving the mellitic acid unaltered. Nitric acid, even when assisted by heat, neither dissolves nor alters mellitic acid. In this respect it bears a striking resemblance to oxalic acid. When a solution of mellitic acid in absolute alcohol is boiled for some time, it would appear from the observations of Wöhler that the nature of the acid is quite altered. When evaporated to the consistence of a syrup it becomes dark brown, and a sour tasted matter remains resembling resin, and like it insoluble in water, though it communicates a sour taste to that Chap. I. liquid. When water is left upon this matter for some time a white tasteless substance remains which burns easily with flame like resin, leaving charcoal, but yielding no sublimate. It is scarcely soluble in hot water, but very readily soluble in alcohol. The solution reddens litmus paper. It dissolves also very readily in liquid ammonia. Atomic weight. From the analysis of mellate of alumina by Wöhler, its constituents appear to be 2-25 is the weight of an atom of alumina, 6·842 is very near 6.75, the weight of 6 atoms of water, therefore 6·624 must be very nearly the weight of an atom of mellitic acid. We may therefore take 6·5 as affording a near approximation to the true atomic weight of this acid. From the great permanence of this acid, and its resemblance in many particulars to oxalic acid, it is highly probable that like that acid it contains no hydrogen, but is a compound of carbon and oxygen. Oxalic acid is a compound of 2 atoms carbon and 3 atoms oxygen. Is it not probable that mellitic acid is a compound of History, This can only be determined by an analysis of mellitic acid. Should it turn out to be free from hydrogen, it will require hereafter to be placed among the acids with a simple base, and next to oxalic acid, to which it has so striking an analogy. The salts which this acid forms with bases have received the name of mellates. SECTION IV.- -OF TARTARIC ACID. Tartar, or cream of tartar, as it is commonly called, when pure, has occupied the attention of chemists for several centuries. Duhamel and Grosse, and after them Margraff and Rouelle the Class I. Div. II. younger, proved that it was composed of an acid united to potash; but Scheele was the first who obtained this acid in a separate state. He communicated his process for obtaining it to Retzius, who published it in the Stockholm Transactions for 1770. It consisted in boiling tartar with chalk, and in decomposing the tartrate of lime thus formed by means of sulphuric acid. The process employed at present for obtaining tartaric acid, Preparation. which is the same with that of Scheele, is the following: dissolve tartar in boiling water, and add to the solution powdered chalk till all effervescence ceases, and the liquid does not redden vegetable blues. Let the liquid cool, and then pass it through a filter. A quantity of tartrate of lime (which is an insoluble white powder) remains upon the filter. Put this tartrate, previously well washed, into a glass cucurbite, and pour on it a quantity of sulphuric acid equal to the weight of the chalk employed, which must be diluted with water. Allow it to digest for 12 hours, stirring it occasionally. The sulphuric acid displaces the tartaric; sulphate of lime remains at the bottom, while the tartaric acid is dissolved in the liquid part. Decant off this last, and try whether it contains any sulphuric acid. This is done by dropping in a little acetate of lead; a precipitate appears, which is insoluble in acetic acid if sulphuric acid be present, but soluble if it be absent. If sulphuric acid be present, the liquid must be digested again on some more tartrate of lime; if not, it is to be slowly evaporated, and about one-third part of the weight of the tartar employed is obtained of crystallized tartaric acid. Lime may be substituted for chalk in this process. In that case the decomposition of the tartar is complete; whereas by Scheele's method, the excess of acid only combines with the chalk; but when lime is used, the whole tartrate of lime by no means separates. A considerable portion is retained in solution by the potash of the tartar now disengaged. If the liquid be evaporated, this portion appears under the form of a transparent jelly. By exposure to the air the potash attracts carbonic acid, which unites to the lime, while the tartaric acid combines again with the potash. To obtain the potash in a state of tolerable purity, the best method, according to Vauquelin, to whom we are indebted for these observations, is to evaporate to dryness, and heat the residue to redness. By lixiviating the mass, the potash will be obtained in a state of considerable purity.* The crystals of this acid are usually so irregular that it is * Ann. de Chim. xlvii. 147. |