ON ORGANIC COLOURING MATTERS. 483 



They have remained sealed up in the tube, in an atmosphere of the gas in ques- 

 tion, since July 9, 1847, when the experiment was made. No decided change 

 was observed in the tints of the papers up to July 30, when I ceased to make 

 notes of their appearance. The rhubarb paper is now little altered, but the blue 

 has, here and there, a few small red spots, some with white borders upon them. 

 Both tints, however, are still, after so many months' exposure to the gas, very 

 slightly affected.* I shew the Society also tubes containing blue paper, which 

 were exposed for five minutes to dry sulphuretted hydrogen on the 25th March 

 1848, and have since remained shut up in the gas. They exhibit, at the present 

 date (April 17), no sensible change in tint.f Sulphuretted hydrogen, then, has 

 its bleaching power arrested by depriving it of water. 



IV. Action of the Acid Gases and of Ammonia on Organic Colours. 



I pass now to the acid gases and ammonia, which so characteristically alter 

 the tints of organic colouring matters, when water is present. The gases I tried 

 were, sulphurous acid, carbonic acid, sulphuretted hydrogen, and hydrochloric 

 acid, in addition to the volatile alkali. 



Sulphurous Acid. 



I need not say anything further concerning sulphurous acid, as it is implied 

 in what was stated as to its negative action on vegetable blues, that its redden- 

 ing action is as much arrested as its bleaching one by depriving it of water. Dry 

 sulphurous acid, I also find, does not change alkalised turmeric or rhubarb paper 

 to yellow. 



Carbonic Acid. 



I have already referred to the retention of its full blue tint, by dry litmus- 

 paper, exposed for many months to a mixture of anhydrous carbonic and sul- 



* The papers have not sensihiy altered after the lapse of two additional months ; nor is there 

 any change in the litmus-paper referred to in the next experiment, June 19, 1848. 



t One of the arguments in favour of the " Binary Theory of Salts" is the fact, that the so- 

 called oxygen acids do not affect vegetable colours unless associated with water, which they are 

 assumed to decompose, so as to become by appropriation of its elements hydracids of new radicals. 

 The experiments recorded in the text, however, shew that one hydracid, at least, has its action on 

 colouring matter as much negatived by the withdi-awal of water as any oxyacid. It does not follow 

 that the rationale of the change is the same in both cases ; but the fact that anhydrous gaseous 

 hydrosulphuric acid does not redden vegetable blues, lessens the value of the argument alluded to. 

 In the great majority of cases, the rendering of an oxyacid anhydi-ous implies its alteration from the 

 liquid state to the solid or gaseous one. This change in condition is of itself sufficient to alter most 

 materially the influence of a reagent. It appears, however, to have been altogether overlooked in 

 explaining the indifference of a di-y oxyacid to organic colours. The phenomena recorded further 

 on, as observed with gaseous hydroclJoric acid and liquefied sulphurous acid, bear upon this point, 

 but it cannot be discussed at length here. 



VOL. XVI. PART IV. 6 H ■ 



