light, or
shows but a feeble result of this action, may, when placed in
proximity with another gas or vapour, exhibit vigorous, if not violent
action. The case is similar to that of carbonic acid gas, which,
diffused in the atmosphere, resists the decomposing action of solar
light, but when placed in contiguity with chlorophyl in the leaves of
plants, has its molecules shaken asunder.

Dry air was permitted to bubble through the liquid nitrite of butyl,
until the experimental tube, which had been previously exhausted, was
filled with the mixed air and vapour. The visible action of light
upon the mixture after fifteen minutes' exposure was slight. The tube
was afterwards filled with half an atmosphere of the mixed air and
vapour, and a second half-atmosphere of air which had been permitted
to bubble through fresh commercial hydrochloric acid. On sending the
beam through this mixture, the tube, for a moment, was optically
empty. But the pause amounted only to a small fraction of a second, a
dense cloud being immediately precipitated upon the beam.

This cloud began blue, but the advance to whiteness was so rapid as
almost to justify the application of the term instantaneous. The
dense cloud, looked at perpendicularly to its axis, showed scarcely
any signs of polarisation. Looked at obliquely the polarisation was
strong.

The experimental tube being again cleansed and exhausted, the mixed
air and nitrite-of-butyl vapour was permitted to enter it until the
associated mercury column was depressed 1/10 of an inch. In other
words, the air and vapour, united, exercised a pressure not exceeding
1/300th of an atmosphere. Air, passed through a solution of
hydrochloric acid, was then added, till the mercury column was
depressed three inches. The condensed beam of the electric light was
passed for some time through this mixture without revealing anything
within the tube competent to scatter the light. Soon, however, a
superbly blue cloud was formed along, the track of the beam, and it
continued blue sufficiently long to permit of its thorough
examination. The light discharged from the cloud, at right angles to
its own length, was at first perfectly polarised. It could be totally
quenched by the Nicol. By degrees the cloud became of whitish blue,
and for a time the selenite colours, obtained by looking at it
normally, were exceedingly brilliant. The direction of maximum
polarisation was distinctly at right angles t