« PředchozíPokračovat »
cess which forms the main subject of the present investigation, much new light was thrown upon the latter. When sulphuric acid, for example, is digested upon turpentine or resin, the oxygen of the former uniting with the hydrogen and carbon of the latter, severally, leaves the acid in the state of sulphureous acid, and produces water and coal. If the process is carried on gradually, before the coal is obtained, we get successively yellow resin, brown resin, and black resin. Now, Mr Hatchett digested with nitric acid, first turpentine itself, then these three resins obtained from the treatment of turpentine with sulphuric acid, and lastly, the coal which is formed at the close of the same pro
Neither the turpentine, nor the yellow nor the brown resin, gave any of the tanning substance ; the black resin yielded a considerable portion of it, and the coal a great abundance. Other substances, reduced to coal by the same process, yielded the tanning residuum with equal facility; as various woods, copal, amber and wax. Nor is the treatinent with nitrous acid the only method of obtaining this tanning substance, from coals made in the humid way. For after resins have been long digested with sulphuric acid, so as to give coal, if they are then digested with alcohol, a solution is obtained, which consists, in part,
of the substance in question. Our author ingeniously conjectures, that some process of this sort takes place naturally in several formations of peat; and thus accounts for the complete tanning which is fréquently found to go on in those mosses where animal bodies have been buried.
The substance resembling tannin, which our author's experiments yielded, was employed with perfect success in tanning skins. This operation he effected with ease, by means of sawa dust, pit-coal, wax-candie, part of the skin itself, and various other materials, not hitherto suspected of subserviency to such a process.
The only property in which this artificial substance seems at first sight to differ from tannin, is, that the former is produced by means of nitric acid; whereas, the latter is destroyed by it. In order to examine the particulars of this diversity, our author instituted several experiments. The artificial substance was subjected to repeated distillations with nitric acid, and found to remain unchanged. The precipitate of gelatine by the artificial substance, was washed and dried, and then digested in nitric acid, which acted powerfully on it, and formed a dark solution with evolution of nitrous gas. The solution being evaporated to dryness, and dissolved in water, was found to have the same action on isinglass, acetite of lead, &c. as the substance which had not undergone these operations. The solutions of this sub
stance, are also quite indestructible by the putrefactive process. Nor do they become mouldy like those of galls, sumach, &c. But are there no varieties in point of destructibility among the different sorts of natural tannin? Our author thinks that his experiments entitle him to answer this question in the affirmative. For having compared, in this respect, the solutions obtained from galls, sumach, oak-wood and oak-bark, he found that the last was much less destructible by treatment with nitrous acid, than the other three, though the artificial product was still less affected by the acid than any.
The artificial substance hitherto considered, owes its origin to digestion of nitric acid upon charcoal, whether of animal, vegetable, or mineral bodies. But it appears that a similar product may be obtained from the exhibition of this acid to bodies not in a charred or coally state ; and that the process of charring only assists the production of the tanning substance, by favouring the separation of the carbonaceous particles from the other ingredients of the mass., Our author ascertained this important point by well conducted experiments, first with indigo, a body known to contain a large portion of carbon. Nitric acid acted violently upon this substance; and the application of a gentle heat was employed to evaporate the solution: The residuum being dissolved in water, was found to contain a large tincture of the tanning substance; other bodies, though in a less degree, yet sensibly yield the same product, by repeated digestions and distillations with nitric acid. Thus, common resin, stick lac, balsam of Peru, benzoin, balsam of Tolu, all gave the tanning substance by digestion with nitric acid; and from these experiments, and others which he details with a variety of different bodies, eur author is induced to think, that there are very few vegetable substances from which a quantity of the tanning compound may not be obtained, by means of the same acid properly exhibited to them.
Many vegetable substances yield, by decoction, after being roasted, a liquor resembling tannin; but Mr Hatchett found it very difficult to apply the precise degree of heat which is required to develop whatever tannin they may contain. Even coffee did not yield a precipitate till some time had elapsed, and then it was soluble in boiling water ; but what he calls pared from the chicoric root,' gave a precipitate, soluble indeed in boiling water, but reproduced when the water cooled. All these decoctions, when treated with nitric acid, yielded a tanning substance similar to that obtained in the former course of experiments.
From the whole it opears, that the artificial substance resembling tannin may be produced in three several ways; by digestion
of nitric acid on any carbonaceous substance, animal, vegetable, or mineral; by distilling the same acid from indigo, resins, and various other substances ; by treating resin, camphor, &c. with alcohol, after they have previously been digested with sulphuric acid. Of these three methods, the first is that which produces the substance most easily, and in the greatest abundance, a hundred grains of vegetable charcoal yeilding about 116 of the substance.
The properties of the body in question are justly deemed by Mr Hatchett to be very singular. Not only its entire resemblance to tannin, except in its relation to nitric acid, but several other particulars in its habitudes, merit our notice. If a portion of this substance, procured from pure vegetable charcoal, be exposed dry to the action of heat, it emits an odour resembling that of burnt feathers, horn, and other animal bodies. If it is exposed in a retort to the distilling process, a red heat sends off the superfluous moisture, and nitrous fumes also rise into the retort; when the heat is pushed higher, a white cloud suddenly fills the apparatus, saline matter is found on the retort, and gas escapes, almost wirh explosive force. This is evidently caused by the evolution of ammoniacal gas, which unites with the nitrous fumes, and forms, as usual, a cloud of nitrate of ammonia. Hence the artificial substance consists of hydrogen, oxygen, and nitrogen, with a base of carbon ; and, as our author had taken every precaution to exclude impurities from the charcoal employed in his experiments for obtaining the substance upon which he operated, he was led to infer, with much confidence, as a corollary from the experiments last mentioned, that hydrogen exists in charcoal; but further consideration, he admits, had induced him to suspend this judgment; for he has always found the dilution of the nitric acid employed in his experiments favour in an eminent degree the formation of the tanning substance ; and this would seem to indicate, that the hydrogen which goes to the composition of that body owes its origin to the water.
The different varieties of this substance exhibit some little dic versity of properties. That which is obtained from carbonaceous bodies, by digestion with nitrous acid, is much more abundantly produced than either of the other species; the precipitates which it yields with gelatine are of a deep brown colour, while the colour of the second variety is bright yellow; both these varieties are more powerful in their action upon skin than the third, which, moreover, seems not to contain any nitrogen ; at least, such is our author's inference from considering the manner of its formation; and by this deficiency in its composition he thinks
its inferior strength may be explained. But upon these matters he promises a further communication.
We have little to add upon the substance, as we have now given it, of these speculations. Their importance can as little be questioned as their general accuracy and solidity. We are disposed to view the discovery of Mr Hatchett as one of the most valuable that have been for many years presented to the chemical world, whether we view it as introducing us to the knowledge of a new body, or as shewing how, in all probability, a known substance is elaborated by Nature in the process of vegetation. By prosecuting these interesting inquiries, we are sanguine in our hopes that much light will be thrown upon the physiology of plants ; and entertain very
little doubt that Mr Hatchett is on the eve of an improvement, perhaps the most valuable that has been made in the useful arts in modern times.
ART. VI. Journal des Mines, publié par l'Agence des Mines de
la Republique. No. I. 8vo. Paris. An 3. THE "He plan of this periodical publication, the mass of important
and accurate information which it contains, and the laudable zeal for the prosecution of scientific and practical mineralogy which we trust it will excite in our own country, have induced us to give a short account of it from the commencement of the series. It cannot be expected, however, that we should dwell on many of the articles separately. Those, for example, which require the illustration of plates, or are already too compressed, or are copied from other works, or which, from their nature, are not susceptible of abridgement or analysis, we shall not regard as belonging to our cognizance. Thus we are compelled to pass in silence various descriptions of machinery ; some excellent crystallographical papers by Haüy, &c.; delineations of subterraneous workings ; sections of local stratification, the condensed forms of several valuable memoirs ; interesting analytical results obtained by Klaproth, Vauquelin and others, and which are now generally known through other channels; various extracts and translations from the transactions of learned bodies, and from foreign or domestic publications ; and such communications as are of a strictly technical or tabular aspect.
As our intention is rather to give our readers an idea of the plan on which the work proceeds, and to invite those whom it may concern to peruse its contents, than to scrutinize its merits with critical minuteness, we shall even abstain from the mode of disquisition in reporting such articles as have chiefly attracted our
attention, attention, and shall, in most cases, be contented with intimating their object and principal results.
In the most stormy and eventful period of the French revolution, the Committee of Public Safety instituted a Board of Mines, consisting of three members. Subordinate to them were eightinspectors, twelve engineers, and forty pupils. The annual salary of the inspectors was fixed at 6000 livres, of the engineers at 3000, and of the pupils at 1500, not including travelling expenses. The inspectors and engineers were proposed by the members of the Board, and approved by the Committee of Safety; and the nomination of the pupils was determined by a public comparative trial. Inspectors, engineers, and pupils were enjoined to travel eight months in the year, and to reside the remaining four at Paris. Two pupils were assigned by lot to each inspector and engineer, to accompany them on their professional mission. The French territory, in regard to the mining department, was divided into eight circles. To the annual survey of each circle one inspector and one engineer were appointed by lot; and each of these officers examined the same circle for two successive years. The four supernumerary engineers were destined to replace such of their colleagues as happened to be indisposed, or employed by government in other functions,
The principal duties required of the inspectors and engineers in their mineralogical tours, were the visitation of mines, the communication of useful advice to the directors of the works, and inspection of the machinery, &c. with regard to solidity and the safety of the workmen. They were, moreover, instructed to visit founderies, and all establishments connected with the mining art, and to favour the directors of such establishments with suitable hints and suggestions. They were particularly desired to instruct their pupils, and to give them practical lessons on all matters connected with their profession. Besides collecting specimens of all the fossil substances contained in the circle, and forwarding the collection, with proper catalogues, to the Board at Paris, they were requested to sketch plans of their discoveries ; to detail the practical operations which obtain in founderies and manufactories connected with mining; to make drawings of machines, furnaces, and of the modes of working under ground; to keep a register of their travelling stations, and of the substances which they found, and the experiments which they made at each; and to transmit their memoirs, journals, and drawings, once in ten days, to the Board. When they discovered any fossil substance which could be turned to good account, they were to invite the proprietors of the ground, or, in their default, the nearest inhabitants, to work it, and to afford them every encouragement, as