16. Dr Bancroft ascertained, that a muriate of tin, containing only half the quantity of the metal which the acid is capable of dissolving, operated even more suc cessfully in dyeing scarlet, than an equal quantity con taining tw ice as much tin; and he therefore concludes, that nearly one half the tin which the scarlet dyers com monly dissolve with aquafortis, and a little sea salt, is wastefully employed; a fact, he adds, which, consider ing the increased price of tin, may, by proper attention, produce a saving of very considerable importance. Ile found also, that the same quantity of tin dissolved in muriatic acid, combined with about one-fourth of its weight of oil of vitriol, forming what he calls a murio sulphate of tin, produced excellent effects as a mordant. The proportions which he found to answer the purpose best, were about 14 ounces of tin in a mixture of two pounds of oil of vitriol (of the usual strength), with about three pounds of muriatic acid of the specific gra vity of 1.17. The muriatic acid should be first poured upon a large quantity of granulated tin, in a capacious glass receiver, and the oil of vitriol afterwards added slowly ; and these acids being mixed, should be left to saturate themselves with tin, which they will do in a longer or a shorter time, according to the temperature of the atmosphere, without any artificial heat. The murio-sulphate of tin, prepared according to the above directions, is perfectly transparent and colourless, and will probably remain so for many years. It will produce fully twice as much effect as the dyers' spirit, or nitro muriatic solution of tin, at less than a third of the ex pence. It has also the property of exalting the colours of all adjective dyes, says Dr Bancroft, more than the dyers' spirit, and folly as much as the tartrite of tin, without changing the natural crimson of cochineal to wards the yellowish hue; and therefore he concludes, after having made a great number of experiments with it. I think myself warranted in strongly recommend ing the murio-sulphate of tin, for dyeing the compound scarlet colour from the cochineal and crimson, and quer citron yellow." 217. Some chemists pretend that the muriate of tin answers the purposes of a mordant most efficaciously when the metal is most highly oxidated; but the experi ments of Dr Bancroft, and the results obtained by the writer of this article, lead to the very opposite conclu sion. The experience of practical dyers is also at vari ance with that opinion ; for it is well known, that when the muriate of tin has been long kept, and thus had time to absorb more oxygen, it acts less powerfully in fixing and exalting colours. Constant observation teaches them, that a less lively and agreeable colour is obtained when the solution is made with rapidity, and the disengage ment of much vapour, than when it is conducted slowly ; doubtless, because in the former case the metal is more highly oxidated. On this account, fresh solutions made ordinary caution, are preferable to old ones, how ever carefully prepared.
218. The tin employed in the preparation of the mor dants, which have for their bases the oxides of that me tal, should be of the utmost purity; common being frequently adulterated with copper and lead, each of which is prejudicial to the colour. Malacca, and good English tin, are reckoned the best; though even the latter generally contains a little copper in combination with it. The presence of iron and copper may easily be detected, by pouring a little prussiate of lime into the solution: the precipitate is blue if iron be present, and of a bronze colour if it contains copper.
219. The muriatic acid should also be in the purest state in which it can be procured. When it is unconta minated with foreign matter, it is perfectly limpid and colourless ; but it is frequently of a light yellowish ap pearance, owing chiefly, it is supposed, to the presence of a small quantity of iron derived from the sea-salt, used in procuring the acid. It may he obtained pure, by subjecting it to a second distillation, from a retort con ndcted with a range of Woulfe's bottles, a little muriatc of soda having been put with it into the retort, and a gentle heat applied.
220. Chaptal has recommended a method of forming the muriate of tin, by causing the vapour of muriatic acid, as it is disengaged from the muriate of soda, to pass through a succession of large receivers, containing granulated tin, with a little water, to absorb the acid. The heat produced by the absorption of the vapour is sufficient to promote a solution of the metal without the expence of fuel. If the receivers be constructed and adapted to each other, upon the principles of Woulfe's apparatus, the process may, in this way, be conducted with a considerable saving of cxpence, as no loss will be sustained by the evaporation of the acid, or of the tin in combination with it, which takes place to a considera ble extent, according to the ordinary method of effect ing the solution.
221. The salts which have already fallen under con sideration, constitute the principal mordants used in dyeing, though several other substances arc occasionally employed for the same purpose. M. D'Ambourney, of Rouen, has made much use of a solution of bismuth. which had formerly been proposed by M. Folic, of the same city. One part of bismuth is dissolved in four parts of nitric acid; the solution is afterwards introduced into a bath containing tartar, and into which is poured, at the same time, a solution of sea-salt. Berth°Het has shown, that in whatever way this mordant is prepared, there is always formed a precipitate by the addition of water, which renders the colouring matter of a brown colour. Chaptal made use of it in dyeing cotton red, for which its author has proposed it; but it was not pro ductive of a greater effect than water acidulated with nitric acid.
222. The oxide of arsenic is also employed as a mor dant in dyeing. Vogler appears to have used with ad vantage a solution of this oxide with potash, in dyeing thread and cotton of a red colour. After dissolving the solution in two parts of water, he added to it a saturated solution of alum. The mixture is at first turbid and gelatinous; but it recovers its transparency, by gra dually adding to it a solution of alum. Thread and cot ton immersed for twelve hours in this mordant, on being washed and dried, assume, with madder, a very deep colour. The sulphurcts of arsenic, known by the names of orpiment, realgar, sandarach, yellow arsenic, red arsenic, Sze. are also used in dyeing, particularly in the preparation of indigo. The sulphuret of antimony may be substituted for that of arsenic; but it does not com municate to the colour the same brightness.
'23. The corrosive sublimate is also used in dyeing. Wilson employed it in the composition of the aluminous mordant for printing linen, in the proportion of one eighth of the sugar of lead. It appears, by the experi ments of Vogler, that this salt renders the colour of madder darker and more durable.