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tinated together by this resinous matter. To separate these filaments from each other, and to destroy entirely the resinous colouring matter, is a process of some difficulty. Solutions of alcalies rendered caustic", called alcaline leys, have the property of
* dissolving resins; hence they have been used for this
purpose in bleaching. The linen is boiled in water containing a quantity of caustic potash, which acts upon the resin of the external filaments, and loosens them a little from each other. The cloth is then spread upon the grass, and exposed to the action of the air, sun, and dew; and is also occasionally watered. It is then returned again into the bucking vat; and the alkaline solution is poured over it: by this another layer of the filaments is opened, and the resin dissolved. It is then carried again to the field, and treated as before. In this manner, the bucking and spreading on the grass are repeated alternately, for 15 or 16 times, according to the weather and other circumstances, until the cloth is whitened. Were the alcaline ley so strong as to dissolve all the resin at once, it would injure the texture of the fabric.
This alternate bucking and exposing on the grass is the old manner of bleaching, and was universally used, till Scheele discovered the properties of the oxygenated muriatic acid in destroying vegetable colours. M. Berthollet first applied this, property to the purposes of bleaching, and he, with great liberality, communicated his observations to the public. For this purpose, he immersed the cloth into diluted oxygenated muriatic acid, between the operations of the alcaline leys, which produced the
* Common potash is rendered sufficiently caustic for the purpose of bleaching, by adding to it quicklime, which has a stronger affinity for the carbonic acid than potash.
same effect in whitening it, as by exposing it to the action of the air and light in the field.
The new method of bleaching was quickly and successfully introduced into the manufactories of France; and almost as soon into those of Great Britain. It is now universally adopted. The advantages are, that the time required for bleaching is shortened in a surprising degree so that manufacturers experience a much quicker return of their capitals; and that it may be carried on at all seasons of the
year. The first way in which the oxymuriatic acid was applied in bleaching was in the liquid state; that is, when water is impregnated with the gas. The goods were immersed in this liquid according to the nature of the objects to be bleached. Skeins of thread were suspended on frames in the tub intended to receive them ; cloth was rolled upon reels. When every thing was thus disposed, the tubs were filled with oxygenated muriatic acid, by introducing a funnel that descended to the bottom of the tub in order to prevent the dispersion of the gas. The cloth, or thread, was made to pass through the liquid by turning the frames, until it was judged that the acid was exhausted by acting on the colouring matter.
But the volatility of this acid, and the suffocating nature of its vapours, which produced extremely noxious effects upon the health of the workmen, rendered its use very difficult, although very ingenious apparatus had been invented both by Berthollet and by Mr. Watts. It was also found difficult to cause the acid to act upon all parts of the cloths equally, when they were stratified in the cisterns with the acid.
A considerable improvement was made in the
apparatus by Mr. Rupp of Manchester, which is described in the Manchester Memoirs. Still it was found that the acid alone was apt to weaken the cloth, and that it injured the health of the workmen.
At length it was discovered by some manufacturers at Javelle, near Paris, that the addition of an alkali to the liquor deprived it of its suffocating effects, without destroying its bleaching powers. Potash was the alcali they employed ; and this solution was called the Javelle liquor. The inventors came into this country, and established a bleaching-work. The process was then carried on in open vessels ; and the bleacher was able to work his pieces in the liquid, and expose every part to its action without inconvenience.
Although these advantages were unquestionably great, they were diminished by the heavy expense of the alcali, which was entirely lost. Also, the the potash, which added to the liquor, though it did not destroy its power of bleaching, diminished it ; because a solution of the oxygenated muriate of potash, which differs from this bleaching-liquor in nothing but in the proportion of alcali, will not bleach at all. This is a well-known fact; from which we might infer, that the oxygenated muri. atic acid will lose its power of destroying the colouring-matter of vegetable substances, in proportion as it becomes neutralized.
Mr. Tennant discovered that lime might be substituted for the potash, the oxymuriatic acid combining with all the alcaline earths, and forming oxy,muriates which were soluble in water, and had the property of bleaching. This is the substance now employed. If the oxygenated muriatic acid be passed through lime-water, it will combine with
the lime, and form oxymuriate of lime; but as the water can only retain a small portion of lime, this was not found of much lise. To cause a larger quantity of lime to combine with the oxymuriatic acid gas, the lime is mechanically suspended by agitation in the water into which the gas is made to pass, so as to present fresh matter to the
gas. By this means, the oxymuriatic acid combines with the lime, forming a compound soluble in the water : this is used as a bleaching-liquor.
The oxygenated muriatic acid gas may also be combined with lime in a dry state. To effect this, theoxymuriatic acid gas is sent into a vessel contain- : ing dry hydrate of lime (that is, lime slacked with water): the powder is agitated, and the gas combines with it to a certain amount, or till the hy: drate of lime becomes saturated. The compound is a soft white powder, possessing little smell. It is partially soluble in water, yielding a solution much the same as that obtained by the former process.
Although most salts that are soluble in water are capable of being formed again by evaporating the water, either in crystals or in a dry saline mass, this is not the case with oxymuriate of lime. Whenever a solution of it is evaporated, part of the acid escapes, and the rest is mostly converted into muriatic acid; so that instead of oxymuriate of lime,
; muriate of lime is obtained. Hence the dry salt cannot be obtained from the liquid solution.
The dry oxymuriate of lime may be very conveniently transported without injury, an advantage not possessed by the acid alone, which cannot be transported without the loss of almost half its strength : but it must be observed that the dry salt is much impaired by being long kept.
We have hitherto used the old term of oxymuriatic acid, because it is best known by this name in the bleaching processes; but it will be remembered that this substance is now considered as a simple body, and is known by the name of chlorine. What has just been called oxymuriate of limę is known among modern chemists by the term chlorate of lime.
The oxymuriatic acid gas, or chlorine, may be procured by distilling muriatic acid in black oxide of manganese; but to save the expense of first preparing the muriatic acid, the usual practice in bleaching is to mix three parts of black oxide of manganese with eight parts of muriate of soda or common salt, and five parts sulphuric acid, diluted with four parts water.
To ascertain the strength of the liquid for bleaching, a solution of indigo in the sulphuric acid is employed. The colour of this is destroyed by the oxygenated muriatic acid; and according to the quantity of it that can be discoloured by a given quantity of the liquor, its strength is known.
The linen is usually not immersed in the solution of oxymuriate of lime until after the fourth or fifth bucking ; because a great portion of the resin is removed cheaper by the alkaline leys, and washing in water.
The last operation in bleaching is souring, or steeping the linen in some sour liquid of a blood heat. For this purpose, formerly sour milk was employed: but now sulphuric acid is used. Of this as much is put into water as will give it the acidity of vinegar. The linens are generally steeped about twelve hours, and are then well washed. This souring is essential to the procuring of a good white, but the theory of its action is not well understood.