ON ORGANIC COLOURING MATTERS. 477 



be refused that character also, for when the latter gas is deprived of moisture, it 

 is more indifferent to colouring matter than even chlorine. Davy's reasoning, 

 then, if pushed to its logical consequences, conducts us to the strange conclusion 

 that since, when moist chlorine bleaches, the chlorine and oxygen are not the 

 active agents, and the only other body present except the colouring matter is 

 hydrogen, which certainly does not decolorise when dry, bleaching must be re- 

 garded as an inexplicable phenomenon, an effect without a cause. 



It would not be difficult to point out other objections to the consistency of 

 Davy's opinion ; but those given may suffice to prove that, before the analysis of 

 the products of chlorine-bleaching shewed the erroneousness of his conclusion, 

 its inherent untenableness admitted of easy illustration. 



II. Influence of Sunlight on the Bleaching Action of Dry Chlorine. 



The chief object of the experiments which this paper details was, to ascertain 

 the cause of the indifference of chlorine when free from moisture to anhydrous 

 organic colouring matters. But before entering on this inquiry, it occuiTed to 

 me to doubt, whether Davy's original proposition, that dry chlorine does not 

 bleach dry colours (which seems to have been universally assented to by his suc- 

 cessors), could be admitted without limitation. 



A repetition of his experiments appears, at first sight, to justify uncondition- 

 ally his conclusion. Among the specimens which accompany this communica- 

 tion are two sealed tubes, containing blue and red litmus-paper shut up in an 

 atmosphere of chlorine. The paper was first dried in a current of air previously 

 passed through chloride of calcium. A stream of carefully desiccated chlorine was 

 then sent over the paper for five minutes, and the tube, whilst full of gas, sealed 

 at the blow-pipe. The coloured papers were thus exposed, in the first place, to 

 the bleaching action of some sixty cubic inches of chlorine ; and have, in addi- 

 tion, remained in contact with that gas since the 28th of July 1847, a period 

 of more than eight months, yet they still retain their original tints, though 

 somewhat faded. Had water been present in these experiments, the colours 

 would have been in-ecoverably destroyed in a few seconds, or minutes at the 

 farthest. 



Striking as these results are in supplying confirmation of Davy's views, they 

 are curiously contradicted, or rather qualified, by other experiments, differing, 

 from those just mentioned, as to mode of trial, only in one particular. 



The affinity of chlorine for hydrogen, when both gases are free, is greatly mo- 

 dified by the action of sunlight, so that whilst in perfect darkness they may be 

 kept mingled without combining, they unite with explosion if exposed to the 

 direct rays of the sun, and more or less rapidly in diffuse daylight, according to its 

 intensity. So faithfully, indeed, do free chlorine and hydrogen obey what natural 



