486 DR GEORGE WILSON ON THE ACTION OF THE DRY GASES 

A, Tube in which coloured paper was placed. 
a, Narrow canal, communicating by a caoutchoue collar, with small tube containing chloride of calcium. 
b and c, Lateral appendages or horns, communicating with long drying tubes, C and D. 
B, Small drying tube, dipping into oil, through which the elastic fluids, after passing over the paper, 
escaped into the air. 
C, Long drying tube, which was sealed at ¢, at the commencement of an experiment, but opened when 
the paper had been dried, and employed to convey the gas which was to act on it. 
D, Second long drying tube employed to carry air, to render the paper in A anhydrous. When the 
paper was dried, the horn c was sealed at the blow-pipe, and D detached. Gas was then trans- 
mitted through C. 
The arrangement I have described was followed with all the gases except 
ammonia. Chlorine and sulphurous acid were sent through oil of vitriol before 
reaching the chloride of calcium tube. The other gases were generally passed 
simply through a bulb immersed in a freezing mixture, before being transmitted 
through the drying tube. I found it essential to success in the experiments re- 
corded to dispense with corks, asbestos, and cotton, in connecting or loosely stop- 
ping the tubes, as these bodies retained moisture with the greatest obstinacy. 
Caoutchouc collars were used in every case to unite the detached portions of the 
apparatus. The special devices followed in particular cases are mentioned under 
the gases which called for them. 
I procured hydrochloric acid gas by the action of Nordhausen sulphuric acid on 
common salt previously fused. Theoretically, the gas should carry with ita 
‘mere trace of moisture, yet in spite of the apparently effectual drying apparatus 
made use of, the acid, in the majority of trials, changed the tint of blue litmus 
as soon as it came in contact with it, giving it a dark lilac or deep wine-red 
colour. Nevertheless, on three occasions I was able to pass a current of dried 
hydrochloric acid for five minutes over blue litmus-paper, without sensibly alter- 
ing its tint, according to the judgment of four persons besides myself, who were 
witnesses of the experiments. These positive results outweigh the negative 
ones already referred to. The majority of the latter, moreover, were only partial 
failures, and went the length, at least, of proving that the removal of moisture 
from hydrochloric acid gas delays, if it does not prevent, its characteristic action 
on organic colours. In the successful experiments mentioned above, the negative 
action of the acid was only transient; for when the tubes containing the 
gas and paper were sealed and set aside, the colour invariably passed from its — 
original blue tint to a more or less decided red. In several trials, however, the 
ultimate effect of the hydrochloric acid fell far short of the full reddening which 
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