. By repeated boiling in alkaline leys the cloth is rendered whiter. But it is not yet bleached ; be cause a colouring matter still remains, ,which the alkalies are incapable of dissolving. The action of oxymuriatic acid, or exposure to the air and light, produces a change on this colouring matter, and renders it capable of being dissolved in alkalies. In short, by these processes it is converted into a resi nous matter, similar to that which the alkalies had previously removed.. There can be no doubt that this change is produced by the union of the colour• ing matter with oxygen. When the oxymuriatic acid is used, the oxygen is supplied by the water which is decomposed by the mutual action of the chlorine and the colouring matter. The chlorine combines with the hydrogen of the water, and is, con verted. into muriatic acid ; while the colouring mat ter combines with the oxygen of the water, and is converted into resin.
It would appear, that this change in the colou ring matter renders it white ; for linen will appear bleach ed if it be treated with a sufficient quantity of oxy muriate of lime. But this state of the colouring , matter is not permanent. If it be allowed to remain in the cloth it speedily becomes yellow. Hence the reason why cloth, bleached with oxymuriatic acid alone, soon loses its white colour again. This hap pened to Berthollet, in his first trials to bleach in the large way, by means of oxymuriatic acid. It hap pened likewise, at first, to several bleachers in this country. It is requisite, that the colouring matter, now become soluble, should be removed by alkalies. Hence we conceive, contrary to the practice of bleischers, that the last process ought always to be boiling in an alkaline ley. In great towns, as in London, where linen cannot be exposed to the air and sun upon the grass, it would be a great advan tage, if it were steeped, for some time before it is washed, in a solution of oxymuriate of lime. It
might then be boiled in an alkaline ley. Linen or cotton, thus treated, would not become yellow by -age, as is . too often the case with linens in large towns.
The precise use of the steeping of the goods in sulphuric acid has not been ascertained ; though it is known to be indispensable. It is supposed, that both linen and cotton contain a portion of iron, and that the acid removes this substance, which both renders the colour whiter and the cloth fitter for the subsequent processes of dyeing and calico-printing.
This explanation is not improbable, though we are not aware of any accurate experiments, by means of which, the presence of iron in the sulphuric acid solution, employed as a souring, has been ascertained. But, probably, the great use of the acid is to remove or neutralize the alkali, which, if allowed to remain in the cloth, would gradually injure its texture.
We have taken no notice, in the preceding article, of a proposal, made a good many years ago in Ire land, and, in support of which, a pamphlet was pub lished by Mr Higgins of Dublin ; we mean the sub stitution of the hydrogureted sulphuret of lime for the alkali. The reason of this omission is, that we are not in possession of any facts on the subject. But we consider the circumstance of no British bleacher having introduced this substitute into his work, as sufficient to entitle us to infer, that the sub stitute would not answer the purposes for which it was recommended. Several objection's to its use will readily present themselves to those who consider the subject. Among others, we may mention, that if any metallic substance, as iron, were to come in contact with the goods which are under the in fluence of hydrogureted sulphuret of lime, this last substance would act as a mordant, and fix the metal on the cloth ; from which it could not be again re moved without some expense, and without the risk of injuring the strength of the substance. (J.)