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 anhydi'ous 

 than even hydrochloric acid ; not so much, perhaps, because it has a greater affi- 

 nity for watei', but because our most powerful desiccating agents, such as the 

 dehquescent chlorides and oil of vitriol, cannot be employed to dry it. We are 

 restricted, accordingly, to substances much less hygrometric, such as unslaked 

 Ume, hydrate of potass, and its fused carbonate. 



It would serve no purpose to record a series of unsuccessful experiments : 1 

 merely mention, therefore, that I have never been able to obtain ammonia in a 

 condition in Avhich it did not change the tint of reddened litmus and of yellow 

 turmeric paper as soon as it came in contact with them. I have 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 hberty 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 tlian chlorine to water for their power of modify- 

 ing colouring matters, both as regards changing and destroying their tints. Hy- 

 di'ochloric 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 wliole 

 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 hav- 

 ing then- 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 Avhich 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. Tlie gas was then allowed to flow through a tube for some 

 minutes till it luid expelled the air, and the tube was sealed. The one en<l of this tube ha<l been 

 previously expanded into a large ball, which was fdled 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- 

 tiimed in the text, immediately became purple, and after some hours bright blue. 



