Chemical gazette - 1850 Author:William Francis Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: THE CHEMICAL GAZETTE. No. CLXXV.—February I, 1850. SCIENTIFIC AND MEDICINAL CHEMISTRY. On Tannic Acid, Gallic Acid and Metagallic Acid. By Prof. G. Mulder... more ». The composition of tannic acid is generally represented by the formula CI8 HJ O + 3HO. From the followmg investigation however it results that this acid should have the formula C38 H'O " + HO, and that it is consequently isomeric with gallic acid. Long ago Pelouze discovered that quercitannic acid is decomposed by heat in the same way as gallic acid. When tannic acid is exposed to a gradually increasing temperature, it loses its hygroscopic water at 248 F. From this temperature to 356 it remains unaltered, but then begins to undergo decomposition. If the temperature is raised to 482, some acid water passes over; carbonic acid is disengaged ; a sublimate, pyrogallic acid, is obtained ; whilst a black substance, to which Pelouze gave the name of metagallic acid, is left in the retort. 1-254 grin. of pure tannic acid, dried at 248, was heated in a closed test-tube, and the gas disengaged collected over mercury. The receiver contained 84 cub. centim. of air at 44-6 F. and 762 millims. After the heating, which was continued for several hours, the gas amounted to 88 cub. centim. at 44-6 and 761-5 millims. after the carbonic acid had been removed by potash. Besides carbonic acid, therefore, the 1-254 tannic acid had disengaged at 4-82, 4 cub. centim. of gas. Tannic acid is not known in a perfectly pure state. According to Pelouze4s method of preparation, a small quantity of a coloured substance, probably metagallic acid, remains with the tannic acid; but this cannot give rise to the 4 cub. centim. of gas, nor can it proceed from any possible admixture of gallic acid. The products of decomposition by a temperature n...« less