The bleaching process, as at that time performed, was very tedious, occupying a complete summer. It consisted in steeping the cloth in alkaline leys for several days, washing it clean, and spreading it upon the grass for some weeks. The steeping in alkaline leys, called bucking, and the bleaching on the grass, called crofling, were repeated alternately for five or six times. The cloth was then steeped for some days in sour milk, washed clean, and crofted. These processes were repeated, diminishing every time the strength of the alkaline ley, till the linen had ac quired the requisite whiteness.
For the first improvement in this tedious process, which was faithfully copied from the Dutch bleach fields, manufacturers were indebted to Dr Home of Edinburgh, who proposed to substitute water, acidu lated with sulphuric acid, for the sour milk previous ly employed. This suggestion was in consequence of the new mode of making sulphuric acid, contriv ed some time before by Dr Roebuck, which reduced the price of that acid to less than one-third of what it had formerly been. It is curious, that when this change was first adopted by the bleachers, there was the same outcry against its corrosive effects as we have seen some years ago, when oxymuriatic acid was substituted for crofting. No allegation, how ever, could be worse founded, and it was completely destroyed by the publication of Dr Home (Essay on Bleaching), who demonstrated the perfect innocence and the superior efficacy and cheapness of sulphuric acid, when properly applied, over sour milk. Ano ther advantage resulted' from the use of sulphuric acid, which was of the greatest importance to the merchant. A souring with sulphuric acid required at the longest only twenty-four hours, and often not more than twelve ; whereas, when sour milk was em ployed, six weeks, or even two months, were requi site, according to the state of the weather. In con. sequence of this improvement, the process of bleach ing was shortened from eight months to four, which enabled the merchant to dispose of his goods so much the sooner, and consequently to trade with so much less capital.
The bleaching art remained in this state, or near ly so, till the year 1787, when a most important change began to take place in it, in consequence of a discovery which originated in Sweden about thir. teen years before.. In the year 1774, there appear ed in the Memoirs of the Royal Academy of Stock holm a paper on manganese, by Mr Scheele. Among other experiments to which he subjected this mine ral, he mixed it with muriatic acid, put the mixture in a retort, and applied heat. He perceived a smell
similar to that of aqua regia. This induced him to collect what came over in a receiver, and he found it to be muriatic acid, altered in a remarkable manner by the action of the manganese on it. Its smell was greatly heightened, it was become less soluble in wa ter, and it possessed the property of destroying those vegetable colours on which it was allowed to act. M. Berthollet repeated the experiments of Scheele on this new acid in 1785, and added considerably to the facts already known. He showed that this new acid (called by Scheele dephlogisticated muriatic acid) is a gas soluble in water, to which it gives a yellowish green colour, an astringent -taste, and the peculiar smell by which the acid is distinguished. When water, impregnated with this acid, is exposed to sunshine, it gradually loses its colour, while, at the same time, a quantity of oxygen gas is disengaged from the water. If the liquid be now examined, it will be found to contain, not the new acid, but com mon muriatic acid. This experiment Berthollet considered as exhibiting an analysis of the new acid, and as demonstrating that it is a compound of muri atic acid and oxygen. On that account, he gave it the name of oxygenated muriatic acid, which was af terwards shortened into oxymuriatic acid, an •appel lation by which it is still known among bleachers.
The property which oxymuriatic acid possesses of destroying vegetable colours, led Berthollet to sus pect, that it might be introduced with advantage in to the art of bleaching, and that it would enable practical bleachers greatly to shorten their processes. At what time these ideas first struck his mind, we do not exactly know ; but at the end of a paper on dephlogisticated muriatic acid, read before the Aca demy of Sciences at Paris in April 1785, and pub lished in the Journal de Physique for May of the same year (Vol. XXVI. p. 325), he mentions that he bad tried the effect of the acid in bleaching cloth, and found that it answered perfectly. This idea is developed still farther in a paper on the same acid, published in the Journal de Physique for 1786. In 1786, he exhibited the experiment to Mr Watt, who, immediately upon his return to England, commenced a practical examination of the subject, and was ac cordingly the person who first introduced the new method of bleaching into Great Britain.