OF HIGH BEFE AN GIBILITT UPON GASEOUS MATTER. 
351 
From the draft of the present memoir, written last February, I take a few notes 
indicative of the difficulties caused by small impurities. Wishing to set my mind at 
rest with regard to nitric acid and hydrochloric acid, I operated for a time upon these 
substances unmixed with any vapour. 15 inches of air which had been permitted to 
bubble through aqueous nitric acid were sent into the experimental tube. The decom- 
posing beam was first sent through a stratum of the liquid acid an inch in thickness. It 
screened the vapour effectually ; no visible decomposition was produced. In this case, 
at the beginning of the experiment, there were a few scattered particles in the tube. 
The cell containing the liquid acid was removed, and a minute afterwards a delicate 
blue colour began to shed itself among the floating particles. It augmented in intensity 
for five minutes, but during that time it could be entirely quenched by the Nicol, the 
particles floating in the blue being left intact. 
These floating particles (mechanically carried in) extended only about 6 inches down 
the experimental tube. Beyond them was a streak of fine actinic blue perfectly polar- 
ized, and beyond this again a dusky grey cloud, which showed no trace of polarization. 
After ten minutes’ action the cloud had assumed a fair density, but it suggested doubts 
whether it was due purely to the nitric acid or to the interaction of the acid and some 
accidental impurity. The experiment was repeated four times with substantially the 
same result. In all cases the beam when passed through the liquid acid proved powerless; 
but always on the removal of this screen, or on displacing it by a cell of water, an action 
was manifested. To all appearance the nitric acid alone generated an actinic cloud. 
The experiments, however, did not quite set my mind at rest. The tube was cleansed 
and the stopcocks heated to redness. When subsequently exposed the nitric acid required 
a much longer time to develope a cloud. After five minutes’ exposure with no cell 
interposed the faintest blue cloud was visible. After ten minutes’ exposure the cloud, 
at first seen with difficulty, was evident for some distance down the tube. By the com- 
plete removal of residues and by strict attention to the cerate employed to make the 
tubes air-tight, the action thus lessened was caused finally to disappear. In each of the 
experiments with nitric acid recorded in the following pages the acid itself was first tried, 
and not until its perfect visible inertness had been proved was it permitted to mix with 
the vapour. 
I also wished to set my mind at rest regarding the action of hydrochloric acid. Several 
experimental tubes were sponged with soap and hot water, washed with alcohol, and 
finally flooded with hot water. They were then thoroughly dried and mounted. On a 
first trial most of them showed a feeble actinic action, which on a second trial usually 
disappeared. In one case the light generated a fine blue cloud which stretched throughout 
the entire length of an experimental tube 3 feet long. One whitish spot only of the 
cloud discharged imperfectly polarized light. The cloud could be utterly quenched by 
the Nicol, with the exception of a small patch of residual blue about 2 inches long, 
which was left curiously suspended in the general darkness of the tube. 
On thoroughly cleansing with dry air the tube containing the cloud, and trying the 
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