484 DR GEORGE WILSON ON THE ACTION OF THE DRY GASES 
phurous acid gases. Pure carbonic acid is equally negative in its action on vege- 
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, ev. 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,—I 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, indifferent 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 
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