ON ORGANIC 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 
atime his change occurred I cannot precisely say, as absence from town be- 
tween the Ist 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 from 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 (the 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 agree 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 well be wished, so that it is not in the direc- 
* The papers shut up with chlorine, and kept in darkness, have not become bleached by two 
months’ longer retention in the gas June 19, 1848 
VOL. XVI. PART IV. 6G 
