Upon M. Seguin's principle, a patent was some years since taken out by Mr. W. Desmond, who obtains the tanning principle by digesting oak-bark, or other proper material, in cold water, in an ap. paratus nearly similar to that used in the saltpetre works. That is to say, the water which has remained upon the pow dered bark for a certain time, in one ves sel, is drawn off by a cock, and poured upon fresh tan. This is again to be drawn oll; and poured upon other fresh tan ; and in this way the process is to be con tinued to the fifth vessel. The liquor is then highly coloured, and marks, as Mr. Desmond says, from six to eight degrees on the hydrometer for salts. He calls this the tanning lixivium. The criteri on to distinguish its presence is, that it precipitates glue from its aqueous solu tion, and is also useful to examine how far other vegetable substances, as well as oak-bark, may be suitable to the pur pose of tanning. The strong tanning liquor is to be kept by itself. It is found, by trials with the glue, that the tanning principle of the first digester which re ceives the clear water is, of course, first exhausted ; but the same tan will still give a certain portion of the astringent principle, or gallic lixivium, to water. The presence of this principle is ascer tained by its striking a black colour when added to a small quantity of the solution of vitriol of iron, or green copperas. As soon as the water from the digester ceases to exhibit this sign, the tan is exhausted, and must be replaced with new. The gallic lixivium is reserved for the purpose of taking the hair off from hides. Strong hides, after wash ing, cleaning, and fleshing, in the usual way, are to be immersed for two or three days in a mixture of gallic lixivi urn, and a thousandth part, by measure, of dense vitriolic acid. By this means the hair is detached from the hides, so that it may be scraped off with a round knife. When swelling or raising is re quired, the hides are to be immersed for ten or twelve hours in another vat, filled with water, and one five hundredth part of the same vitriolic acid. The hides be ing then repeatedly washed and dressed, are ready for tanning ; fin which purpose they are to be immersed for some hours in a weak tanning lixivium, of only one or two degrees ; to obtain which, the latter portions of the infusions are set apart, or else some of that which has been partly exhausted by use in tanning. The hides are then to be put into a stronger lixivium, where, in a few days, they will be brought to the same degree of satu ration with the liquor in which they are immersed. The strength of the liquor will by this means be considerably di. minished, and must therefore be renew ed. When the hides are by this means completely saturated, that is to say, per fectly tanned, they are to be removed, and slowly dried in the shade. Calf skins, goat-skins, and the like, are to ne steep ed in lime-water, after the usual fleshing and washing. These are to remain in the lime-water, which contains more lime than it can dissolve, and requires to be stirred several times a-day. After two or three days, the skins are to be removed, and perfectly cleared of their lime by washing and pressing in water. The
tanning process is then to be accom plished in the same manner as for the strong hides ; but the lixivium must be considerably weaker. Mr. Des mond remarks, that lime is used, in stead of the gallic lixivium, for such hides as are required to have a close grain ; because the acid mixed with that lixivium always swells the skins more or less: but that it cannot, with the same convenience, be used with thick skins, on account of the considerable labour re quired to clear them of the lime ; any part of which, if left, would render them harsh, and liable to crack. He recom mends, likewise, as the best method to bring the whole surface of the hides in contact with the lixivium, that they should be suspended vertically in the fluid, by means'of transverse rods or bars, at such a distance as not to touch each other. By this practice, much of the labour of turning and handling may be saved. Mr. Desmond concludes his specification by observing-, that in some cases it will be expedient to mix fresh tan with the lixi vium ; and that various modifications of strength, and other circumstances, will present themselves to the operator. He affirms that, in addition to the great sav ing of time and labour in this method, the leather, being more completely tan ned, will weigh heavier, wear better, and be less susceptible of moisture, than lea ther tanned in the usual way ; that cords, ropes, and cables, made of hemp or speartery, impregnated with the tanning principle, will support much greater weights without breaking, be less liable to be worn out by friction, and will run more smoothly on pullies ; insomuch that, in his opinion, it will render the use of tar in many cases, particularly in the rigging of ships, unnecessary ; and, lastly, that it may be substituted for the preservation of animal food instead of salt. The intelligent manufacturer will viaddy perceive, that this new method is grounded on two particular circum stances, besides a more scientific ma nagement of the general process than has been usual. The first consists in the method oldetermining the presence and quantity of the tanning principle by the hydrometer, and the precipitation ofglue: the second in applying this principle, in a concentrated state, more early in point of time than has, perhaps, been hitherto done. Our tanners, after the common previous processes, and unhairing by acids, by lime, or by piling the hides, that they may heat and begin to putrify, apply the solution of tan, which they call ooze, in a great number of pits in the tan-yard.. They begin with the weakest solution which has been used, and is of a lighter colour than the other ; and they pass the hides, according to their judg ment and experience, into oozes which are stronger and stronger ; until at last, in certain cases, the hides come to be buried, for a certain time, in a solid mass of tan, or oak-bark. The oak-bark itself, in the pits, is not only the source from which the water extracts the tanning principle, but seems, likewise, in some measure, during the last stages of the process, to operate mechanically, by keeping the surfaces of 'the hides from touching each other.