ON ORGANIC COLOURING MATTERS. 481 



more manageable process for preparing it, by means of which we may hope to 

 make researches as to its action on colours.* My experiments have been limited 

 to sulphm-ous acid, sulphuretted hydrogen, and oxygen. I begin with the last as 

 the chief rival in bleaching power of chlorine. 



Oxygen. 



I have not thought it necessary to make many experiments with oxygen, as to 

 its relative bleaching power when moist and dry. Test papers can be preserved 

 for years, without sensibly changing tint in air, i. e., diluted oxygen, only mode- 

 rately dry, especially if free exposure to light be avoided. The general experience 

 of mankind has led to the same conclusion, in reference to the comparative per- 

 manence of tint, of dyed tissues kept in the shade. I have exposed coloured 

 papers for four and five hours to a current of dry air, without permanently alter- 

 ing their hue. The paper in such trials always exhibits a duller tint at the 

 end than at the beginning of the experiment ; but that this is the result merely 

 of its loss of water, is evident from the fact that, on moistening the paper, 

 the original brightness of tint is restored. No one, probably, will dispute the 

 conclusion, that dry oxygen does not, at least in darkness, bleach more than dry 

 chlorine. 



The effect, on the other hand, of the addition of water to oxygen in increasing 

 its decolorising power, is so strikingly demonstrated by the practical experience 

 of the domestic bleacher, that experiments on the small scale did not seem neces- 

 sary to prove the fact. No point is more attended to, in the familiar practice of 

 bleaching cloth by free exposure to rain, wind, and sun, than the constant keep- 

 ing of the tissue Avet. I am far from affirming that other important agencies, 

 such as the actinic, concerned in the bleaching, are not affected by the presence 

 of water ; yet I think no one will doubt, that one important function it serves, is 

 the increasing (I do not at present say how) the bleaching action of the oxygen of 

 the atmosphere. 



The unquestionable decolorising power of peroxide of hydrogen, chromic, and 

 permanganic acids, to which Dr Turner refers as confirming Davy's view re- 

 garding the bleaching action of moist chlorine, only demonstrates that nascent 

 oxygen bleaches, and is of no service in proving that that gas, when in its state 

 of perfect elastic fluidity, possesses bleaching powers. The nascent hydrogen of 

 decomposing water bleaches readily ; so that, if Dr Turner's view were accepted 

 as valid in relation to oxygen, a theory of chlorine-bleaching might, with some 

 plausibility, be defended, in which hydrogen, instead of, or as well as, oxygen, 

 should be represented as the positive bleaching agent in chlorine- water. 



* It would be peculiarly interesting to observe the effect of drying this gas in modifying its action 

 on colouring matters. Should it lose its bleaching power when dry, it would be curious to watch the 

 effect of exploding it in the presence of an anhydrous colouring matter. The result would shew whether 

 nascent oxygen and chlorine bleach as powerfully when dry as moist. 



