Independently of the weakening of the power of the acid by this addition, a con siderable expense was introduced by the use of the alkali ; and it became an ob ject of importance to the manufacturers of this country to substitute a cheaper substance, which should have the same effect. Lime was tried at first in an im perfect manner, but at length with such improvements, that it is now always used.
The difference of using it arose from the insolubility of the lime in water, the glum thy taken up being so inconsiderable, that the solution could have little effect in cor recting the odour of the acid. A very important improvement, therefore, was that of using lime suspended in water, and kept in suspension by an agitation in a close vessel, into which the gas was transmitted. Its condensation was thus facilitated, and the compound which is formed with the lime being soluble in water, the undissolved or unsaturated lime was allowed to subside, and the clear liquor was fit for the purpose of bleaching.
An improvement, however, of still more importance, has been made by Mr. Tennant of Glasgow, and a patent obtain ed for it ; viz. that of combining the oxy muriatic acid with dry lime, and dissolv ing a certain proportion of this compound in water, to form a bleaching liquor. It perhaps could scarcely have been sup posed that such a combination could have been formed, so as to retain the powers of the acid. But the trial has filly suc ceeded, and the advantages derived from it are important ; the compound can be carried easily to a distance, and the ma. nufacturer need not prepare it himself, which is always an advantage, especially where he does not work on a large scale. The combination is formed by introducing the oxymuriatic acid gas through leaden tubes into slacked lime, prepared from chalk, by which it is absorbed. Solutions of this are prepared of different strengths, according to the purposes to which they are to be applied, the strength being judged of by the hydrometer, and by the quantity requisite to destroy the colour of a diluted solution of indigo in sulphu ric acid. The process of bleaching, as now performed by these liquors, differs little from that which has been already described as executed by the solution of the oxymuriatic acid alone in water. To these methods, however, is to be added the more recent discovery of bleaching by an alkali, assisted by a watery vapour and a high temperature, and which, either alone, or combined to a certain extent with the method by the oxymuriated acid, is now practised with so much advantage. In this method, which has been long in use in some of the eastern countries, and of which notice was first given by Chap tal, the cloth or thread is impregnated with a solution of potash or soda, render ed active by the carbonic acid having been entirely abstracted from the alkali by lime ; it is suspended loosely, and, with an extensive surface, in a close boil er, a quantity of the same solution being in the bottom, and heat is applied, the boiler being closed, with a safety valve in the cover, so that the vapour under pres sure may receive a high temperature.
It is kept in this situation for a number of hours. The thread or cloth when cold is washed, and either exposed on the field, or subjected to the action of the oxymuriatic acid in some of the forms under which it has been used. It is thus at once rendered perfectly white. The superiority of this method probably arises from the high temperature, and the solvent power of the watery vapour, fa vouring the action of the alkali on the colouring matter, while this vapour pene trates the fibres of the cloth so effectual ly, that the matter is in a great measure dissolved and removed.
The animal fibres that are subjected to the bleaching process, are wool and silk. These cannot be treated in the same manner as vegetable substances : a strong alkaline ley will dissolve them, and oxy muriatic acid will both weaken them and turn them yellow. The colour of manu factured wool resides partly in its own oil, and partly in the greasy and mucila ginous applications which it receives in being prepared for the loom. Both the one and the other are easily got rid of, by the action of fuller's earth and soap in the process of fulling. Puller's earth is a very fine-grained absorbent earth, which by itself is capable of mixing rather than combining with vegetable or animal oils, and rendering them miscible with water : its action is found, however, to be increased by the addition of soap ; and woollen cloth being beat in a fulling. mill with hot water, and a proper mix ture of earth and soap, or of soap alone, and afterwards well washed and dried in the air, receives all the bleaching which it requires, or is indeed capable of. It is then of a white colour, somewhat verg ing towards yellow : this last tinge may be made to disappear, by the addition of a very small quantity of stone blue in the water in which the cloth is last washed, or by exposing it to the fumes of burning sulphur. By this latter method, how ever, it acquires a certain harshness of feel, and is apt to turn very yellow when washed with soap. Both the colour and harshness of raw silk depend entirely on a yellow varnish with which it is naturally covered. This varnish may be in part removed by long boiling in simple water. It is considerably more soluble in alco hol ; but the most effectual and expedi tious way of clearing is by putting it in a i linen bag, and boiling it some hours in a solution of white soap and water, then rincing it in clean water, and repeating the process till it is quite white, and ex hibits the peculiar lustre of this beautiful substance. Some of the French chemists have endeavoured to lessen the con sumption of soap, by proposing various substitutes ; but nothing is so effectual and expeditious as the purest white soap, and the article itself is so valuable, as amply to repay this expense.