482 DR GEORGE WILSON ON THE ACTION OF THE DRY GASES 
Sulphurous Acid. 
The bleaching power of moist sulphurous acid is so well known, and has so 
long been turned to account in the arts, that I need enter into no details in proof 
of the gas possessing this property. With a view to determine whether it loses 
its bleaching power when made anhydrous, I passed washed sulphurous acid, ob- 
tained by the action of mercury on oil of vitriol, through strong sulphuric acid, 
and over chloride of calcium, so as to deprive it of moisture. The gas was then 
made to stream for five minutes through a tube containing blue litmus-paper, 
carefully dried, and the tube hermetically sealed while full of the gas. This ex- 
periment was made on March 31, 1848. The paper was not altered in tint dur- 
ing the passage of the gas, and at the present date (April 17) it remains un- 
changed.* I made a similar experiment on the 10th of July 1847, only the 
sulphurous acid, being obtained by the action of charcoal on oil of vitriol, was 
mingled with carbonic acid. The mixed gases were passed dry over blue litmus- 
paper for seven minutes, but did not change its tint in the slightest; the tube 
was then sealed, and is included among the specimens laid before the Society. 
After the lapse of nearly nine months, the paper continues not appreciably altered. 
Gaseous sulphurous acid, then, is no better bleacher when dry than chlorine. 
Sulphuretted Hydrogen. 
The other remarkable properties of sulphuretted hydrogen have prevented its 
bleaching power from attracting very much attention; nevertheless, it has long 
been recognised. Nascent sulphuretted hydrogen bleaches powerfully. An aci- 
dulated infusion of litmus has its colour rapidly destroyed by the addition to it of 
a metallic sulphuret, such as that of calcium, barium, or iron. The free gas 
bleaches much less distinctly, yet its action is tolerably rapid. In proof of this, 
I have sent with this communication a tube, which, after the blue litmus-paper 
contained in it had been dried, was opened for a few seconds to the atmosphere, 
in consequence of a derangement of the apparatus. The paper was thus exposed 
for a very short period to the amount of vapour which is diffused through air at 
the temperature of about 60° F. - Immediately after this accident, carefully-dried 
sulphuretted hydrogen was passed over the litmus for five minutes, and the tube 
sealed. The paper was distinctly, though faintly, reddened during the passage of 
the gas, and after the lapse of about twenty hours the colour was found almost 
completely gone. In contrast with this result obtained with slightly moist sul- 
phuretted hydrogen, I shew the Society a tube containing dry blue litmus-paper, 
and brown-red rhubarb paper, which were exposed to the action of equally dry 
sulphuretted hydrogen for six minutes. The papers were not altered in tint. | | 
* The colours are still unaltered ; a remark which applies also to the experiment next recorded, 
June 19, 1848. 

