BLEACHING 365 



Us place by an act of substitution. Now, whether the chlorine or the liberated 

 oxygen removes the hydrogen, the result will bo the saine the destruction of the 

 compound. Chlorine so readily performs these changes, that we should at once decide 

 on calling it the active agent, wore it not for the fact that oxygen acts so readily, even 

 when chlorine is not present : for example, peroxide of hydrogen, as well as the oxides 

 just mentioned, and ozone also, which has no chlorine to help it. It is, then, certain 

 that oxidation bleaches ; and it is certain that dehydration bleaches, if performed by 

 chlorine, and that the sun aids it by its active rays. We know also that water aids 

 it : water aids bleaching or oxidation by air, partly because it contains air in solution. 

 It aids also the bleaching performed by solutions in contact with porous bodies, be- 

 cause these bodies have a power of condensing gases in their pores and of compelling 

 combinations. The next question is, Does it aid the bleaching by chlorine in the same 

 way, by assisting the union mechanically, or by decomposing water ? Chlorine acts 

 slowly, unless water be present. The theory, therefore, does not demand the decom- 

 position of water, and the known powerful affinities of chlorine do not require to be 

 supplemented by oxygen. But, in order to see exactly the state of the case, let us look 

 at the action of chlorine in hypochlorites or in chloride of lime, and we find that it is 

 a direct oxidation. We obtain by it peroxides of metals, and not chlorides. Here wo 

 seem to be taught directly by experiment, that bleaching by hypochlorites is an 

 oxidation of the colouring-matter. Bleaching by moist chlorine may therefore be 

 looked on as the same ; indeed, we oxidise by it; but in such cases we may obtain 

 the base at the same time united to chlorine, giving another turn to the question, as 

 Kane showed. The oxidation theory, therefore, seems to be sufficient when water is 

 present. We are, however, finally to deal with dry chlorine in the sun ; and in that 

 case it is fair to conclude that it acts by direct combination with hydrogen or the 

 colouring-matter or both. We have, then, two modes of bleaching ; but the usual 

 mode in the air becomes by that explanation an oxidation, and the direct action of 

 chlorine obtainable only with difficulty. When sulphurous acid is used, another 

 phenomenon may be looked for, as we find a substance whose chief quality is that of 

 deoxidising. The removal of oxygen also decomposes bodies, and sulphuretted 

 hydrogen can scarcely be supposed to act in any other way. Sulphurous acid, when 

 it decomposes sulphuretted hydrogen, really acts as an oxidising agent, and we can 

 therefore imagine it as such in the bleaching process. Investigation has not told us 

 if it enters into combination as SO 2 , and, like oxygen, destroys colour, altering the 

 compound by inserting itself. 



We may fairly conclude that the processes by chlorine and sulphurous acid are 

 performed in a manner as different as the mode in which a salt of ammonia acts on 

 chlorine or an oxacid, or, in Dr. Wilson's general terms, ' specific differences may be 

 expected to occur with all the gases named, as to their action on any one colouring- 

 matter, and with different colouring-matters, as to their deportment with any one of 

 the gases.' Trans. E. 8. K, 1848. 



It has been attempted to introduce manganates, chromates, chlorates, chlorochromic 

 acid, and sulphites, but without success, as bleaching agents. 



BLEACHING OF COTTOK. 



Substances dealt with in Bleaching. The object of bleaching is to separate from 

 the textile fibre all the substances which may mask its intrinsic whiteness, or, which, 

 in the course of dyeing or printing, may produce injurious effects on the colours. 

 The substances present in cotton goods, and to be treated in bleaching, are as follows : 



a. The resinous matter natural to the filaments. 



b. The colouring-matter of the plant. 



c. The paste of the weaver. 



d. A fatty matter. 



e. A cupreous soap. 



f. A calcareous soap. 



g. The filth of the hands. 



h. Iron rust, earthy matters, and dust. 



t. The cotton fibre itself. 



j. The carbonaceous matter caused by singeing. 



k. The seed-vessels. 



a. Cotton is covered with a resinous matter, which obstructs its absorption of 

 moisture. This alone would prevent it receiving colour, and it is known that if this 

 could be removed, some of the darker colours could bo dyed without any bleaching, 

 providing also the impurities arising from manipulation wore absent, although the 

 finest colours could not bo produced in this manner on cotton in general. M. Bolly, 



