484 DB GEORGE WILSON ON THE ACTION OF THE DRY GASES 



phurous acid gases. Pure carbonic acid is equally negative in its action on vege- J 



table blues and browns. 



Sulphuretted Hydrogen. 



Sulphuretted hydrogen, even when moist, does not change organic colours to 

 so great an extent as the stronger acids do. Solutions of litmus, ex. gr., become, 

 under its action, only of a purple-red tint, like that which carbonic and boracic 

 acids give them, whilst the more powerful acids destroy all shade of blue. If I 

 may judge from the few experiments I have made on this subject, the reddening 

 power of sulphuretted hydrogen is more dependent than its bleaching action on 

 the presence of water. At all events, it is equally dependent on moisture, for 

 blue litmus has been reddened very slightly by eight months' exposure to the dry 

 gas, neither has brown rhubarb paper become yellow, or appreciably grown paler. 



Hydrochloric Acid. 



No acid excels hydrochloric in full and rapid action on organic colours ; nor is 

 any one, according to the prevailing opinions of chemists, less likely to be indebted 

 to association with water for its characteristic properties. It is the simplest type 

 of a perfect acid, and as such, might be expected to exhibit, even when gaseous 

 and anhydrous, the same relation to organic colours which it does when moist. 

 I looked upon hydrochloric acid, therefore, as the most interesting of the acid 

 gases with which experiments could be made. 



I have not hitherto referred particularly to the method followed for drying 

 the gases, because none of those I have yet mentioned present great difficulties 

 in the way of rendering them, — 1 will not say certainly anhydrous, — but at least 

 sufficiently dry not to affect colours. It is otherwise with hydrochloric acid. I 

 have failed more frequently than I have succeeded, in rendering this gas, by dry- 

 ing, indiiferent to colours ; nor have I been able to preserve blue litmus for any 

 length of time unchanged in an atmosphere of the dry gas. It is necessary, there- 

 fore, to be more particular in describing the process for drying, which was fol- 

 lowed with hydrochloric acid ; although it differed in no respect from that pur- 

 sued with the majority of the other gases. 



The general arrangement, especially in the later and more perfect trials, was 

 the following: — The thinnest India letter-paper was stained with an infusion 

 or tincture of the colouring matter intended to be used, and afterwards dried at 

 the temperature of the air. Slips of the paper were introduced into a tube, vary- 

 ing in different cases from half an inch to one inch in diameter, and from six to 

 eighteen inches in length. The tube was then hermetically sealed at one extre- 

 mity, and drawn out at the other into a narrow canal, which was left open. A 



