History. Pinic acid The pinates of barytes and soda deviate so far from the others that we cannot include them. The mean atomic weight of pinic acid derived from the others is 66-95. We may, therefore, pitch upon 67, as not far from the truth. Of Colophonic Acid. Colophon is the name given on the continent to the brown, brittle, semitransparent resin, which remains when turpentine has been subjected to distillation in order to obtain oil of turpentine. Unverdorben first showed that it was capable of neutralizing alkaline bodies, and therefore possessed properties analogous to acids. Indeed as it has been long employed in this country in the manufactory of soap, this inference might have been very naturally drawn, though I am not aware that any chemist in this country actually drew it. From Unverdorben's experiments it appears that pinic acid when strongly heated becomes brown, and is partially converted into colophonic acid. Colophonic acid seems, therefore, very closely connected with pinic acid; and Unverdorben has nowhere pointed out in what the difference between them consists. According to the analysis of Gay-Lussac and Thenard, colophon is composed of which Unverdorben, from his experiments on the atomic weight of colophonic acid, has resolved into This would make its atomic weight 75-625. But the atomic weight derived from his analyses of several colophonates comes nearly to the same number as that given in the last section for pinic acid. In fact the data from which that number was got, results from the analysis of colophonates. Now the number of atoms corresponding with the atomic weight of 67, and with the analysis of colophon by Gay-Lussac and Thenard, are 64 atoms hydrogen 67 atoms carbon 9 atoms oxygen 8 50.25 9 67.25 This, therefore, ought to represent the atomic constitution of colophonic acid, according to the data furnished by Unverdorben. M. Unverdorben states that colophon is not pure colophonic acid, but that it is contaminated by a mixture of an indifferent resin. But he has no where given a process by which it may be purified. When thrown down from an alkaline solution by means of an acid, it is in the state of a hydrate, composed of 100 colophon and 13.1 water. This is very nearly 1 atom colophon 8 atoms water 67 9 76 Class L Div. II. SECTION XLI.-OF SILVIC ACID. This acid forms one of the constituents of the common tur- History. pentine which exudes from the pinus silvestris. Hence the name given it by Unverdorben. He considers it as identical with the crystalline substance which Riess met with in white pitch, and likewise with the abietic acid of Baup, mentioned in the last section. It may be obtained pure by the following process: Mix common turpentine with water, and distil to get rid of the volatile oil. Digest the residual matter in cold alcohol of the specific gravity 0.883, which will leave undissolved most of the silvic acid, mixed with a small quantity of pinic acid. Then boil this undissolved residue in twice its weight of alcohol of the specific gravity 0-883. As the solution cools the silvic acid precipitates still mixed with about four per cent. of pinic acid, but from this it may be freed by repeated solutions in the boiling hot alcohol of 0.883. Or if the precipitate be digested repeatedly in cold alcohol of the specific gravity 0.883, the pinic acid will be dissolved, while the greatest part of the silvic acid • Jahrbuch des Polytechnischen Instituts zu Wien, i. 435. Chap. I. will remain unaltered. But the easiest way of obtaining it pure is to dissolve the impure acid in twice its weight of absolute alcohol, containing th of its weight sulphuric acid. When the solution cools the silvic acid shoots into large crystals, free from pinic and sulphuric acid, provided they be washed in alcohol of the specific gravity 0·883. Properties. Silvic acid is transparent and colourless, and has the form of low four-sided prisms, terminated by a four-sided pyramid. When heated it melts into a colourless mass, without any appearance of crystallization .When scratched with a hard body it breaks with a slight crack, as happens also to pinic acid. It is nearly as hard as colophon. When rubbed it becomes electric. When heated it becomes soft and thready. It fuses at the temperature of about 212°. It is insoluble in water, but dissolves in alcohol and ether. When heated to 336° it gives out no water. When raised to a higher temperature than this in a retort, it becomes first thick and then quite fluid, gives out a little water and empyreumatic oil; it then boils and gives out no more water, but is converted into oil at first limpid but becoming gradually thick, while at the same time a little gas is disengaged. If the process be stopped when two-thirds of the matter has passed over the residue is light brown, and consists of a mixture of silvic acid, pinic acid, a little resin similar to pinic acid, and a neutral resin soluble in absolute alcohol, from which it falls when left to spontaneous crystallization in colourless crystalline flocks. When silvic acid is boiled in nitric acid, it slowly undergoes decomposition, being changed into a yellow resin, which when dissolved in alcohol strongly reddens litmus paper. This altered resin possesses also the properties of an acid, and combines with the different bases. Silvic acid dissolves readily in sulphuric acid. The solution is yellow brown, and water throws down from it a mixture of a little silvic acid with a great deal of pinic acid. Silvic acid may be obtained in the state of a hydrate by throwing it down from its solution in potash by means of a stronger acid. The water does not separate unless the silvic acid be mixed with oxide of lead. Unverdorben, to whom we are indebted for all the facts respecting this acid which have been stated, examined also a number of the silrates, and determined their properties. But as he made no attempt to analyze any of these salts, we are still ignorant of the atomic weight of this acid. It is doubtless high. SECTION XLII.-OF CARBAZOTIC ACID. Class I. Div. II. It was discovered by Haussmann in 1788, that when indigo History. is treated with nitric acid, it is converted into a yellow coloured and bitter tasted substance, which has been ever since known by the name of bitter principle of indigo. It was afterwards examined by Fourcroy and Vauquelin.‡ And at a still latter period by Chevreul,§ who endeavoured to show that it was a compound of nitric acid with a peculiar principle derived from indigo. In 1827 an elaborate set of experiments was made on it by M. Liebig,|| who demonstrated it to be a peculiar acid composed of azote, carbon, and oxygen; but without any trace of nitric acid, and to which he gave the name of carbazotic acid. His mode of obtaining this acid in a state of purity was as follows: The best indigo reduced to a coarse powder is digested with eight or ten times its weight of nitric acid of mean strength at a very moderate temperature. It dissolves with a copious emission of nitrous fumes, while it froths up very considerably. After the violent ebullition is over it is raised to the boiling temperature. Then a little more concentrated nitric acid is added, and this is persisted in as long as there is a disengagement of red fumes. When the liquid has cooled there will be deposited a great quantity of yellow-coloured semitransparent crystals; and supposing the process properly conducted, neither resin nor artificial tannin makes its appearance. Wash these crystals in cold water, and dissolve them in boiling water, and crystallize a second time. To obtain the carbazotic acid quite pure dissolve these crystals again in boiling water, and saturate them with carbonate of potash. On cooling carbazotate of potash crystallizes. It is best to dissolve these crystals again in water, and to crystallize them two or three times successively. When the carbazotate of potash is sufficiently pure, dissolve it in water and decompose it by nitrie, muriatic, or sulphuric acid. When the solution cools beautiful crystals of carbazotic acid are deposited in plates. A good deal of the same acid may be obtained from the mother leys by a similar process. Four parts of indigo yield about one part of carbazotic acid. ‡ Phil. Mag. xxiii. 256. • Unverdorben, Poggendorf's Annalen, xi. 393. Kohlenstickstoffsaure. Ann. de Chim. et de Phys. xxxv. 72. Chap. I. Properties. Atomic weight. Carbazotic acid thus obtained is crystallized in fine plates, having a silky lustre and a fine yellow colour. It reddens vegetable blues, and has an exceedingly bitter taste. When heated it melts, and is volatilized without decomposition. When suddenly heated it takes fire and burns with a yellow flame, leaving a residue of charcoal. It is little soluble in cold, but very soluble in hot water, and the solution has a deeper yellow colour than the solid acid itself. It is soluble also in alcohol and ether. It is not acted on by chlorine or iodine. Sulphuric acid has no action on it cold; when hot it dissolves it; but lets it fall unaltered as it cools. Boiling muriatic acid has no action on it whatever, and aqua regia scarcely alters it. These facts are sufficient to show that it contains no nitric acid, as Chevreul supposed that it did. It is rather a powerful acid, uniting with and neutralizing the different bases. The carbazotates have the curious property of detonating when heated, as was long ago discovered by Foureroy and Vauquelin. Liebig examined a good many of the carbazotates, and subjected carbazotate of potash and of barytes to analysis. The following were the results which he obtained: 1. Carbazotate of potash. Carbazotic acid Potash 2. Carbazotate of barytes. Carbazotic acid Barytes 31.01 6 30.41 9.5 The mean of these two analyses gives us 30-71 for the atomic weight of this acid. It was analyzed by Liebig by means of oxide of copper, and found composed of The hydrogen, supposing it to contain that principle, scarcely exceeded 1 per cent, of the weight of the acid. The number of atoms corresponding with this analysis is |