488 DR GEORGE WILSON ON THE ACTION OF THE DRY GASES 
mine is probably sufficiently accounted for by the supposition, that I did not 
thoroughly dry the ammonia. This gas is more difficult to render anhydrous 
‘than even hydrochloric acid; not so much, perhaps, because it has a greater affi- 
nity for water, but because our most powerful desiccating agents, such as the 
deliquescent chlorides and oil of vitriol, cannot be employed to dry it. We are 
restricted, accordingly, to substances much less hygrometric, such as unslaked 
lime, hydrate of potass, and its fused carbonate. 
It would serve no purpose to record a series of unsuccessful experiments: | 
merely mention, therefore, that I have never been able to obtain ammonia in a 
condition in which it did not change the tint of reddened litmus and of yellow 
turmeric paper as soon as it came in contact withthem. Ihave found its action on 
colouring matter, however, sensibly reduced by passing it over the hygrometrics 
last referred to. Reddened litmus, for example, became only purple when it first 
encountered dried ammonia, and did not acquire a bright-blue tint, when left in 
the gas, till after the lapse of some hours.* For the reasons mentioned above, I 
do not, in the meanwhile, feel myself at liberty to say more than that the pre- 
sence of water greatly quickens the action of ammonia on colours. 
It would appear, then, from the results I have detailed, that there is little, if 
anything, anomalous or exceptional in the negative bleaching of dry chlorine. 
Oxygen, sulphurous acid, and sulphuretted hydrogen, are equally powerless as 
bleachers, when deprived of moisture, as that gas. Sulphurous and carbonic acids 
are probably more indebted than chlorine to water for their power of modify- 
ing colouring matters, both as regards changing and destroying their tints. Hy- 
drochloric acid and ammonia have their influence on colours at least temporarily 
arrested by the absence of water; and, after all, it is a question with the whole 
of the gases referred to, only of degree. It is not likely that even in absolute 
darkness chlorine has no action on anhydrous colouring matter. If this be con- 
ceded, the whole of the gases referred to may be included in one category, as hay- 
ing their modifying action on organic colours accelerated by the presence and re- 
tarded by the absence of water. I trust to supply an additional datum towards 
the settlement of this question, by observing the difference which exposure to sun- 
light makes, in relation to the action of all the gases with which experiments were 
tried. 
* In the experiment which yielded the most successful result, the ammonia was first passed 
through a bulb immersed in a freezing mixture, and afterwards through long tubes containing lime, 
caustic potass, and its fused carbonate. The gas was then allowed to flow through a tube for some 
minutes till it had expelled the air, and the tube was sealed. The one end of this tube had been 
previously expanded into a large ball, which was filled with fragments of the hygrometrics just men- 
tioned: in the other end of the tube a small sealed bulb was placed, containing a piece of carefully 
dried red litmus-paper. The ammonia was left in contact with the drying agents for a week, when the 
tube was shaken till the bulb broke, and allowed the gas and the paper to meet. The latter, as men- 
tioned in the text, immediately became purple, and after some hours bright blue. 
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