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XXXIIT—On the Action of the Dry Gases on Organic Colouring Matters, and its 
relation to the Theory of Bleaching. By Grorce Witson, M.D., Lecturer on 
Chemistry, Edinburgh. 
(Read, April 17, 1848.) 
I. Preliminary Remarks. 
The remarkable power which chlorine possesses, of destroying all colours of 
organic origin, has long been an object alike of speculative and practical interest. 
The theory of bleaching, however, has hitherto remained imperfect, in conse- . 
quence chiefly of the observation of Sir H. Davy, that chlorine loses the power of 
decolorising when deprived of water. So striking is the difference, in this respect, 
between wet and dry chlorine, that it led the distinguished chemist mentioned, to 
deny to this gas the character of being essentially, or per se, a bleacher. He re- 
garded as the true decolorising agent the oxygen of the water, which must be 
associated with chlorine before the latter can bleach. The late Dr TurNsr has 
stated Davy’s view so shortly and clearly, that I quote his abstract in preference 
to any statement of my own. ‘ Davy,” says he, “ proved that chlorine cannot 
bleach, unless water is present. Thus dry litmus suffers no change in dry chlo- 
rine; when water is admitted the colour speedily disappears. It is well known 
also, that hydrochloric acid is always generated when chlorine bleaches. From 
these facts, it is inferred that water is decomposed during the process; that its 
hydrogen unites with the chlorine, and that decomposition of the colouring mat- 
ter is occasioned by the oxygen which is liberated. The bleaching property of 
binoxide of hydrogen, and of chromic and permanganic acids, of which oxygen is 
certainly the decolorising principle, leaves little doubt of the accuracy of the 
foregoing explanation.’’* 
The opinions of chemists on this point have now entirely changed. Chlorine 
is reinstated in the place it formerly held as the possessor of positive and intrinsic 
bleaching powers, and is looked upon as exerting these even when water is pre- 
sent. I quote Sir Ropert Kane’s views on this subject, as more fully expressed 
than the statements of most of our chemical authorities, and because he has de- 
monstrated experimentally the falsity of Davy’s conclusion. ‘‘ The theory,” 
says he, in allusion to bleaching, ‘‘of this action of chlorine, which had been 
formerly thought to depend upon a mere oxidation of the colouring matter, water 
being decomposed, has been shewn by my results with orceine, and confirmed by 
those of ErpMAN on indigo, to consist in the formation of new substances con- 
* Elements of Chemistry, 7th edit., p. 275. 
VOL. XVI. PART IV. 6F 
