The bleacher's operations should go on in regular rotation. The whitest goods are put into the clean fresh steeps. As these goods do not exhaust the bleaching power of the liquor, its strength is restor ed after they are taken out, by the addition of fresh stoek-liquor. It is then used for goods in a less advanced state of whiteness. If the second opera tion has not exhausted its bleaching powers, or ren dered it foul, it may be used a third time by another addition of stock-liquor. But if it has been render ed very foul, and indicates only 12 or 15 degrees in the test-tube, it is not worth preserving or using.
One pound of the bleaching powder, as it is pre pared by Mr Tennant and Company of Glasgow, is le of forming from ten to twelve gallons of liquor degree.
The exposure of the goods to the action of the oxymuriate of lime should not commence till after the third boil or buck, if they are to be exposed three times to the action of this substance ; and af ter the second, if they are to be exposed to four ope rations, with muriate of lime. These operations should alternate last bucks or boils. In all cases, the oxymuriate of lime must be dissolved in such a proportion of water as to allow the goods im mersed in it to float loosely and easily in the solution, that the liquid may come into free contact with eve ry part of them.
Though the method of immersion or steeping the goods in the solution of oxymuriate of lime is most generally used, yet, in some cases, great advantage is obtained by wincing the goods through the solu tion, instead of allowing them to rest for any length of time in it. When the method of wincing is fol lowed, a solution of twice or thrice the ordinary strength may be safely used, and much time in con sequence saved. When the goods are coarse and heavy, such as yarn for ducks, this last method is al most universally preferred. These heavy yarns re. quire from Ath to -Nth of their weight of oxymuriate of lime, divided into a number of operations cor responding to that of the boils, and following these boils, or these boils with exposure on the grass, when such exposures are employed. In the first oxymu riatic operation, they are winced through a strong solution, produced by adding about three or four ounces of dry oxymuriate of lime to each gallon of water. The yarns are hung over a roller fixed in the mouth of the trough or tub that contains the solu tion, which covers part of its surface. This roller
being then turned rapidly round by means of a crank fixed on one of its ends, the yarn is made to pass rapidly and repeatedly through the liquor for the space of thirty or forty minutes. Fresh parcels of yarn are, in succession, passed through the liquor, its strength being restored by additions of oxymu riate when necessary, and the whole changed when exhausted of bleaching powers and foul. In this way, a very powerful bleaching effect is produced, and yarns which have undergone one process in this way, may receive the remaining oxymuriatic pro. ceases to which they are to be exposed in the or dinary way of immersion, which is less expensive in point of labour, though less efficacious.
The oxymuriatic process then is repeated three or four times, beginning after the second or third boil or buck, and alternating with every subsequent boil or buck. The time during which the icoods are exposed to the oxymuriatic solution during each process, is from six to twelve hours.
4. The fourth process consists in steeping the goods in an acid solution. - This is called the acid process. We have already observed, that the acid at first used for this purpose was sour milk ; but that, when Dr Roebuck contrived his new method of pre paring sulphuric acid, Dr Home proposed this acid as a substitute for sour milk ; that it speedily came into general use, and very much improved and short ened the tedious bleaching processes at that time in general use. This steeping in sulphuric acid is re peated twice, and sometimes thrice, once after each of the last two or three immersions in the oxymu riate of lime. For this purpose, a quantity of strong sulphuric acid is taken, amounting to about nth or Ath of the weight of the goods to be immersed. This acid is diluted with 60 or BO times its weight of water, which reduces it to the specific gravity of about•1.015. Now, sulphuric acid of this specific gravity contains about 11. per cent. of its weight of real sulphuric acid. In this liquid they are steeped from eight to twelve hours. When linen fabrics are intended for printing, they require two or three additional processes in alkali, and one in acid, and the solution of the sulphuric acid is generally made one-third stronger when the goods are intended for the madder copper.