BLEACHING (Aug.-Sax. blacon, from Mere, pale, bleak) is generally understand to mean the process of whitening-or decolorizing cloth; but the term is also applied to the tlecolorizing of such substances as the fixed oils. Irish moss, etc. Until about the close of toe 18th c., B. depended upon the natural bleaching 11,_geucies present in the atmos phere and in the sun's rays. The usual plan was to spread out the cloth on a grass-field, called a bleaching-green, and to continue sprinkling it with water several times a day. After being thus exposed for several months to the action of air, light, and moisture, the cloth was rendered white. The process was necessarily tedious and occupied much valuable land, and for this reason a large quantity of the cloth required to be bleached was sent to Holland for that purpose. A. particular kind of linen, which was regularly scat to Holland, received on that account the name of lIallands; and another variety Of liaea, from its fineness, was generally spread on the better grass-fields or lawns, received the title of lawn. An improvement in the preceding process was to dip the cloth occasionally in a weak alkaline lye, or solution of an alkali, such as soda in ~cater, which step was called bucking; atter which the cloth was spread out on the grass for some weeks, end regularly moistened with water, this stage being styled crafting; the cloth was then soaked in sour milk and water, which was called souring, and again exposed on grass to the action of air and sunlight. By repeating the bucking, crofting, and souring operations several times. the bleaching was very much hastened, and the amount of land occupied in bleaching-greens lessened. The next improvement was the introduction of dilute sulphuric acid instead of sour milk, as the souring agent; and this was so Clreettlal, that it lessened the time required for B. from about eight months, which was the original time, to about four months.
Till very recently, it was thought that the agent in this natural mode of B. was entirely resident iu the sun's rays, but the discovery of the substance called ozone (q.v.), which possesses very powerful B. properties, and which in greater or less quantity exists in the air of country districts at all times, has led to the opinion, now held by chemists, that the 13. which takes place when the cloth is moistened and exposed to the air is mainly due to the ozone present therein; though the chemical rays which accom pany the luminous rays of the sun may assist in the B., and also aid in the formation of
the ozone. That the ozone has very Inuch to do in open-air B., is observable from the fact that in town districts, where little or no ozone exists in the air, cloth is never bleached white.
In the year 1785, Berthollet, a distinguished French chemist, discovered the power ful B. properties of chlorine (q.v.), and immediately thereafter it was suggested that chlorine would he useful in the B. of cloth. At the first, the gas chlorine was employed, and being diffused in the atmosphere of a vessel or small apartment., cloth hung therein was speedily bleached. It was found, however, that the chlorine, which bleaches, or destroys color by uniting with the hydrogen of the coloring principle and thus decom posing the color, could also unite with the hydrogen of the fibre (see LIGNIN) and destroy or render tender the textile fabric. So long as chlorine was employed in the gaseous state, it was very difficult use it of such strength as only to destroy the color, without also rotting the cloth. It was then suggested, that as chlorine was soluble in water, to the extent of two volumes of chlorine gas in one volume of cold water, the solution of chlorine might be employed. But although chlorine water was found to act efficiently and safely when the solution was of the proper strength, it was very difficult always to make it of the same strength, and more so to preserve it when made; as the least expo sure to light causes more or less of the chlorine to unite with the hydrogen of the water, forming hydrochloric acid. which does not possess B. properties. After attempts to fix the chlorine in alkaline solutions, it was found that dry slaked lime was an admirable absorber of chlorine gas. The material produced from the union of chlorine with dry slaked lime is known as the chloride of lime, or bleaching powder (q.v.), and this is the substance which has continued from 1799 up to the present time to be the great artificial bleacher of cotton and linen fabrics. It is not serviceable in the destruction of the color of wool, silk, or the oils and fats; such materials being bleached by the employment of other agents, as will be afterwards noticed.