of the hydrometer, but are far from being always worthy of confidence. The composition of tannogelatin varies greatly according to the manner in which the precipitate is produced. If made by mixing solutions of tannin of oak-bark and gelatin in such proportions that no great amount of either predominates, it generally consists, when dried at 212°, of 100 parts of gelatin, and about 88 parts of tannin; if made by adding a great excess of a strong solution of tannin to the solution of gelatin, it contains 100 parts of gelatin, and about 118 parts of tannin; but if made with an excess of gelatin, it contains 100 parts of gelatin, and about 60 parts of tannin.* It would be difficult to precipitate the whole of the tannin in this way without applying the gelatin in slight excess, but the precipitate at first formed would then partially redissolve in the excess of gelatin. Some varieties of tannin, moreover, produce much larger precipitates with gelatin than other kinds: infusions of cork-tree bark and cutch, for instance, afford smaller precipitates than an infusion of gall-nuts of similar strength; hence this test would be inapplicable to a tan-liquor made of mixed materials. If the tannin to be estimated is the same kind as that which exists in gall-nuts, a solution of sulphate of quinine may be used as the precipitant much more advantageously than a solution of gelatin. According to M. Larocque, sulphate of quinine precipitates tannin from its solution perfectly, and the composition of the precipitate seems to be constant; It will be perceived that the proportions of tannin in the three precipitates are nearly as 1, 14, and 2, respectively. but different kinds of tannin are also unequally affected by this agent. The test solution may be prepared by dissolving the sulphate of quinine of the shops in water acidulated with a few drops of sulphuric acid. The most ready method of obtaining the strength of a tan-liquor, and the only accurate method which would be frequently practised in a tannery, is an imitation on the small scale of the actual process of tanning, which would afford not merely the proportion of tannin, but that of all the matters in the liquor capable of being absorbed by the hide. A piece of perfectly dry prepared hide or skin, such as the clippings used for making glue, is digested in a given quantity of the liquor, till all the tannin, &c., have been separated, then taken out, slightly washed, dried, and weighed. The increase on its weight is, of course, the weight of the tannin and other matters required. An experiment of this kind may be completed in a few hours if a thin skin be operated on, and the action be assisted by a moderate heat. Care should be taken to have a greater quantity of hide or skin present than is sufficient to combine with all the tannin in the infusion. One principal respect in which the common modern processes of tanning differ from the process generally practised until towards the close of the last century, consists in the mode of preparing the infusion of the tanning material as well as of applying it to the prepared hides. Anciently the infusion was commonly made in the pits where the impregnation of the hides was effected; the ground bark and hides being laid in alternate layers in the tan-pit, which was then filled with water to the brim. When the infusion appeared to be spent, the pit was emptied, and refilled with fresh bark and water; and this operation was repeated whenever the bark became exhausted. This tardy process is still practised to a small extent, and by it leather of superior quality is produced; but to obtain a more rapid impregnation of the hides, the tanning infusions are now almost always prepared at first in a much more concentrated state than they could be obtained when applied as in the old method. The use of the simple aqueous infusion of the tanning material was proposed about the close of the last century by M. Seguin, who recommended that the infusion should be prepared by passing cold water through a stratum of the powdered vegetable substance until the latter, by several successive infiltrations of the same quantity of water, becomes completely deprived of its ingredients, which are soluble in cold water. The manner of applying the infusion thus obtained to the hide will be considered in another section of the present article. A large number of tanners now make their tanning infusions with hot or lukewarm water; the usual source of heat being steam, which is introduced by a large iron pipe to the bottom of a deep rectangular pit containing the mixture of the vegetable material and water. At a little height above the true bottom of the pit is placed a perforated false bottom, through which the infusion filters into the space below, from whence it is withdrawn by a pump. The following mode of preparing a strong tan liquor adapted to the production of leather, by the modern quick process, is found to work well. The water, instead of being mixed with the fresh tanning material, which we may suppose to be bark, is first applied to bark which has already been almost exhausted of its soluble ingredients, and after the mixture has digested a considerable time at a moderate heat, the weak infusion thus obtained is pumped into another pit containing more bark, not so far exhausted as that in the first pit. From the second pit, after a time, the infusion is transferred to another containing richer bark; and so on, through a series of several pits, until it arrives in that which contains the fresh material. From the latter the liquor is withdrawn for use. II. PREPARATION OF HIDES AND SKINS FOR TANNING. In commercial language, all kinds of skins which are operated on by the tanner are arranged into three classes, technically known as hides, kips, and skins. Hides are the skins of large and full-grown animals, such as oxen, cows, horses, bulls, and buffaloes; kips are the skins of the same animals flayed while young;* the name skins is applied by the tanner to those of small animals, as sheep, seals, goats, dogs, &c. Hides are used for making thick and strong sole leather, and kips and skins for the * Most of the kips which are imported, however, are the skins of fullgrown, and often very old animals belonging to a small breed of cattle. This remark applies to those exported from Russia as well as from the East Indies. The skin of a calf which has been out at grass is termed a kip; but if the animal has not been taken to grass, it is considered a skin. thinner and more flexible upper shoe-leather, glove and fancy leather, &c. Hides, kips, and skins may be obtained by the tanner in four different conditions; fresh from recently slaughtered animals, dried, salted, and both dried and salted: the fresh hides are furnished by the home slaughter, the others are imported. Some idea of the relative estimation in which equal weights of the different kinds of hides are held by the British tanner may be formed from the following list, arranged in the order of their value, beginning with those most esteemed: Dry ox and cow hides from Buenos Ayres and Monte Video. Salted ox hides from Buenos Ayres and Monte Video. The highest price of the best Monte Video hides (dry) is generally eightpence per pound (duty paid), and the lowest price current of the worst hides from New South Wales (salted) is twopence halfpenny per pound. Kips are imported from the East Indies, dry, brined and dry salted; from Buenos Ayres, salted; and from St. Petersburgh, dry. Their prices vary from fourpence to one shilling per pound. Sheepskins are mostly imported from Buenos Ayres, and seal-skins from Newfoundland. The price of market hides varies from twopence halfpenny, to fourpence per pound. Though the lowest in price, they are considered the most valu |