ON OEGANIC COLOURING MATTERS. 479 



months. The inclosed papers still retain their original colours little altered ; and 

 in perfect darkness would, in all probability, have retained them still better. 

 Side by side with this tube I have placed its twin, which was exposed to full sun- 

 shine, and the papers in which are bleached to the purest white. In how short 

 a time his change occurred I cannot precisely say, as absence from town be- 

 tween the ] st of August and the 16th of September 1847 prevented me from watch- 

 ing the progress of the actinic bleaching. But, on the last-mentioned date, I found 

 the paper completely decolorised, so that six weeks of sunshine sufficed totally to 

 bleach paper in dry chlorine, whilst that gas excluded ft-om direct sunlight has 

 failed to produce the same effect in eight months and a half.* In another quite 

 similar experiment, the results were much less striking. A tube with dry chlo- 

 rine and litmus-paper has hung since 1st August 1847 in a western exposure, 

 yet, at the date of my writing, (April 13, 1848), the litmus-paper, though much 

 faded, as appears when it is contrasted with the contents of the twin tube which 

 was kept out of sunshine, is far from being entirely bleached. This difference 

 in result leads to the suspicion, that in the experiment first recorded, the chlo- 

 rine or the paper may not have been so dry as both were in the second trial. 

 Great precautions (tlie same in both cases) were taken to secure absence of mois- 

 ture from the gas and the paper, but I know of no test of perfect dryness appli- 

 cable to gases, and I cannot affirm that, in either case, the chlorine or the colour- 

 ing matter was absolutely anhydrous. Nor does it admit of doubt that the pre- 

 sence of even a trace of water would sensibly quicken bleaching under sunlight, 

 which rapidly decomposes chlorine- water. Yet every chemist will acknowledge 

 that chlorine, which could be retained over litmus without bleaching it for 

 nearly nine months, must have made a close approximation to perfect dryness. 

 We are as yet, moreover, too ignorant of the laws and conditions of actinic action, 

 to know well how to dispose of apparent discrepancies in its effects. 



I could not try more than the two experiments recorded, last summer, and I 

 did not think it desirable to attempt a repetition of them during the clouded 

 season of the year. Meanwhile, different as is the testimony these experiments 

 afford, as to the rapidity of actinic chlorine-bleaching, they agi-ee in proving that 

 darkness, as well as dryness, is essential to the preservation of organic colours 

 from destruction by chlorine, and that this gas, at least when assisted by sun- 

 light, is a positive bleacher. Davy's original proposition must be accepted with 

 this qualification. 



I close my remarks on this subject, with the observation, that in bleaching on 

 the large scale it should make a sensible difference on the rapidity of the process, 

 whether it be carried on in open sunlight, or in exclusion from it. Our present 

 bleaching process is as rapid as could weU be wished, so that it is not in the direc- 



* The papers shut up with chlorine, and kept in darkness, liave not become bleached by two 

 months' longer retention in the gas June 19, 1848 



VOL. XVI. PART IV. 6 G 



