308 An Ej/ay on Bleaching. 



we intimatclv acquainted with the natulre of potafli and foua, 

 we might be able to explain the caufe of this burning ; but It 

 is furlieient for us at prcfent to know the ejf'etl. The ex* 

 pofure of vegetable matters on the grafs fubjecls them to 

 the action of the folar rays ; and moiftening them during 

 their expofure facilitates, with the vaporization of the water,^ 

 the emanation of the carbonic acid formed by the oxygen of 

 the atmofphere, which combines with the carbon reiulting 

 from the alkaline eombuvtion. It even agrees pretty well 

 with theory, in terminating the procefs of bleaching, to im- 

 merfe the cloth and thread in lour milk, acidulous liquors, 

 or, what is more convenient, very weak fulphuric acid. 



In proportion as the alkali, during Lie full immerfion, 

 dellrovs the colouring matter, the oxygen of the atmofphere y 

 or that hmnlhcd by the oxygenated muriatic acid, joins that 

 earbonized matter, and forms carbonic acid, which after- 

 wards refolves itielf into gas. This is contained the more 

 in the principles, as the bafes of all the acids are infallible in 

 water; but when the combination takes place between the 

 carbon and the oxygen, it immediately becomes foluble. 

 Thus, on the one hand, to burn the colouring matter, and 

 to diffove it on the other, form the whole fecret of the art 

 of bleaching; and the greater or lefs tendency of vegetable 

 fubftances to experience that com bullion conflitutes the 

 gradations of their whitcnefs, and the facility or difficulty of 

 bleaching. 



The tlownefs of the old proceffes arofe, in a great meafure, 

 from bleachers being unacquainted with thefe principles. 

 A long fuccetfion of leys, and expofure on the grafs, was 

 necciiarv to penetrate ihe fibres of the linen from tlratum to 

 ftratum. The texture was fufficiently clofe to refill the action 

 of the heat of a common ley; and a confiderable time was 

 required to abforb the oxygen prefented by the delicate ftra- 

 tum of atmofpheric air. 



In the proeefs of bleaching by fleam, thefe difficulties are 

 removed. The high temperature of the ileam in the interior 

 part of the apparatus fvvells up the fibres of the thread or 

 cloth ; the pure alkali, which rifes with the elaftic fluid, feizes 

 with avidity on the colouring matter, and burns it: feldom 

 does the tirfue of the flax or hemp refill the penetrating effect 

 of this vapour bath. The whole matter, therefore, by which 

 they are coloured is attacked and decompofed by this fingle 

 operation ; and even if we fuppofe that a part has been able 

 to refill, nothing is neceflary but to repeat the operation, 

 after a previous immerfion and expofure on the grafs, to in- 

 fure its complete effect. The alkali even appears to have 



a much 



