A JOURNAL OF NATURAL PHILOSOPHY, CHEMISTRY, AND THE ARTS. SEPTEMBER, 1809. ARTICLE I. On the Union of Tan and Jelly: by JOHN BOSTOCK, M. D. SIR, To Mr. NICHOLSON. DURING the course of the last spring I was engaged in Purpose of the a set of experiments, which may be considered as a conti- author's inquiry. nuation of those formerly made on the analysis of animal fluids. My object was to enable the operator to apply the tests, which indicate the existence of the principal constituents of these fluids, albumen, jelly, and mucus, so as not only to discover the qualities of the compound, but the quantities of its ingredients. The results of my experiments have been, upon the whole, unsuccessful; and I have at present chiefly to announce the failure of the different expedients, which I employed to attain my object. It may. not, however, be altogether useless, to lay my experience * See Journal, vol. IX, p. 244. VOL. XXIV. No. 106 SEPT. 1809. B before Jelly. pitate by tan ate to the quantity pre sent, before your readers;, not merely because I have it in my power to state some few facts, that may be considered as an addition to our stock of knowledge, but still more, because I may induce some one more skilful than myself, to point out a method of accomplishing what I have hitherto attempted without success. The substance upon which I first operated, and to which I shall principally confine my attention in the present paIts characters. per, is jelly; the characteristics of which are its solubility in water, its forming an insoluble compound with tan, and the property which its aqueous solution possesses of concreting by cold, and being redissolved by the application Inquiry whe- of heat. The problem which I was anxious to solve was, ther the preci- whether the compound of tan and jelly be uniform, so that be proportion- by saturating the gelatinous part of a solution with tan, and collecting the precipitate, we may, from its weight, (the quantity of tan employed being known) ascertain the amount of the jelly previously contained in the fluid. From the experiments that had been performed on the subject, Mr. Biggin's particularly those of Mr. Biggin and Mr. Davy, I conceived, attempt to ascertain the prothat this would be found to be the case. The object of Mr. portion of tan. Biggin's experiments was to ascertain the proportion of tan in different barks, for which purpose he formed similar infusions of them, and precipitated the tan from each by a solution of glue. He employed the solution of glue always of the same strength, and by collecting the precipitates, he judged of the quantity of tan that had united itself to the glue, and thus of the proportion of it in the bark *. The experiments are important, as comparing the different barks with each other, and thus ascertaining their respective value as substances to be employed in the manufacture of leather; but it is obvious, that, unless the compound of tan and glue be uniform, they do not show the absolute quantity of tan in any given weight of bark. Mr. Davy, in his shows the pre experiments on astringent substances, has pointed out, with cipitate is proportionate to his accustomed sagacity, the different effects that are prothe strength of duced in the union of solutions of tan and jelly, according to their degree of concentration; and has proved, that in Mr. Davy the solution. * Phil. Trans. 1799, p. 260. proportion proportion to the strength of the solution, either of jelly or of tan, will be the weight of the precipitate obtained *. It would appear, that, when the solutions are much diluted, the attraction of both the jelly and the tan for the water, to a certain extent, counteracts their attraction to each other, and thus prevents a portion of them from being removed from the fluid. Mr. Davy, however, as well as Mr. Both suppose Biggin, evidently seems to have conceived, that the sub- the preci itate stance which was precipitated in all instances possessed the same properties, and consisted of a uniform compound of the two ingredients. This opinion is the very foundation of the method which he employed in his analyses, and is directly asserted in different parts of his papers t. to be a uniform compound. converse of With this impression it was, that I entered upon a set of The author's experiments, which may be considered as the converse of proceeding the those of Mr. Biggin and Mr. Davy. The object of these theirs. chemists was, by the agency of jelly, to remove all the tan from a vegetable infusion, and to estimate its quantity from the weight of the precipitate; while mine was, by means of tan, to ascertain the quantity of jelly that was contained in any animal fluid. In pursuing this investigation, the first A uniform re point was to determine upon the most proper substance to agent requisite employ as the reagent; for as it is difficult, if not absolutely impossible, to procure tan in a state of perfect purity, it became necessary to discover some vegetable infusion, which should always possess similar properties, and in which the quantity of tan should be known, without having recourse to any long calculation. My attention was naturally, in the first instance, directed to galls; and I expected, that Galis. by employing equal weights, infusing them in equal quantities of water, and for an equal length of time, fluids. would have been formed always containing equal quantities of tan. But upon making repeated trials, I find that this is not the case; and it would appear from all the experience Not uniform in I have had upon the subject, that two parcels of galls will their nature, scarcely ever be procured, which will precisely agree in their *Phil. Trans. 1803. Phil. Trans. 1803. Nicholson's Journal, vol. V, p. 259, 269, & alibi. B 2 nature. nature. If finely powdered galls be infused for two hours in 8 times their weight of boiling water, an infusion is formed, which is generally transparent, of a deep brown colour, and which contains about one tenth of its weight of solid matter. But although this is the usual result of the process, it is by no means constantly so. Frequently the infusion will be thick and muddy, will not be rendered clear by being passed through the filter, nor will it become so after standing at rest for several days; its colour also varies considerably, the brown tinge existing in different shades of intensity, and occasionally being exchanged for a bottle green. The quantity of solid matter contained in the fluid is seldom precisely the same in any two trials; although it is generally about one tenth, yet I have occasionally found it no more than one fourteenth. Although it, may appear at first view somewhat singular, that such different effects should be produced by the same substance; yet, when we attend to the visible difference, that exists in gall nuts, we shall easily conceive how these variations may or homogene take place. The structure of galls appears to have been little attended to, and they have generally been spoken of as homogeneous bodies, before the accurate description of their several parts, that is given by the Mr. Aikins in their late valuable publication*. ous in their structure. An extract of them does not answer. As it appeared impossible to employ a recent infusion of galls for the standard fluid, I thought of evaporating the infusion, and making use of a solution of the dried residuum. But I found, that this residuum, although formed from a perfectly transparent infusion, is not capable of being completely redissolved, owing to some change that has been effected on one or more of its constituents, probably the extract, by which it becomes no longer soluble in water. This circumstance forms an insuperable objection to the employment of the dried residuum as a standard, because the quantity of matter, depending upon the variable proportion of the soluble and insoluble part, or of the tan and extract, will scarcely ever be found the same in any two specimens upon which we may operate. * Aikins' Chem. Dict. Art. Gall nut. The artificial tan The infusion of galls, however prepared, seemed inade- Mr. Hatchett's quate to the purpose of affording an accurate test for jelly, I thought therefore of employing the artificial tan discovered by Mr. Hatchett, because, being a substance formed by a specific chemical action, it may be supposed always to possess the same chemical properties. It was accordingly prepared by digesting powdered charcoal in nitric acid, and the result coincided entirely with the description of Mr. Hatchett; it was readily dissolved both in water and alcohol, it precipitated jelly from its solution, and also the nitromuriate of gold, the muriate of tin, the superacetate of lead, and the oxisulphate of iron. All these properties show its strong resemblance to the infusions obtained from astringent vegetables. I was however disappointed in not finding it to answer the purpose that I had in view. Although the artificial tan very readily afforded a precipitate formed an imfrom a gelatinous solution, yet the jelly seemed to be only perfect preci imperfectly thrown down, the fluid remained muddy after the operation, and the precipitated matter could not be completely separated from it. This circumstance I found to take place with different portions of the artificial tan, which were each of them prepared with every attention to Mr. Hatchett's directions; and, I conceive, depends upon a owing to the quantity of undecomposed acid, which remains attached to presence of un❤ decomposed the tan, and which cannot be entirely removed from it. acid. This excess of acid was always found in my experiments, and must probably have existed in Mr. Hatchett's prepara tions, for he points out their property of reddening litmus as one that is characteristic of them*. To whatever cause we may ascribe it, it seemed to be a sufficient objection to the use of this substance as a test for jelly. pitate, Catechu was next tried, but without any better success. Catechu does Independent of the difference which exists between dif- not answer. ferent specimens of this substance, which is considerably greater than what is found in the infusion of galls, I have never met with any catechu which is entirely soluble in water. In the different trials that I made to procure standard * Phil. Trans, for 1805, p. 215. solutions |
