is shown in fig. 3, where L represents the
electric lamp, ss' the experimental tube, pp' the pipe leading to the
air-pump, and F the test-tube containing the volatile liquid. The
tube tt' is plugged with cotton-wool intended to intercept the
floating matter of the air; the bent tube T' contains caustic potash,
the tube T sulphuric acid, the one intended to remove the carbonic
acid and the other the aqueous vapour of the air.

The power of the electric beam to reveal the existence of anything
within the experimental tube, or the impurities of the tube itself, is
extraordinary. When the experiments is made in a darkened room, a
tube which in ordinary daylight appears absolutely clean, is often
shown by the present mode of examination to be exceedingly filthy.

The following are some of the results obtained with this arrangement:

Nitrite of amyl. The vapour of this liquid was in the first instance
permitted to enter the experimental tube, while the beam from the
electric lamp was passing through it. Curious clouds, the cause of
which was then unknown, were observed to form near the place of entry,
being afterwards whirled through the tube.

The tube being again exhausted, the mixed air and vapour were allowed
to enter it in the dark. The slightly convergent beam of the electric
light was then sent through the mixture. For a moment the tube was
_optically empty_, nothing whatever being seen within it; but before a
second had elapsed a shower of particles was precipitated on the beam.
The cloud thus generated became denser as the light continued to act,
slowing at some places vivid iridescence.

The lens of the electric lamp was now placed so as to form within the
tube a strongly convergent cone of rays. The tube was cleansed and
again filled in darkness. When the light was sent through it, the
precipitation upon the beam was so rapid and intense that the cone,
which a moment before was invisible, flashed suddenly forth like a
solid luminous spear. The effect was the same when the air and vapour
were allowed to enter the tube in diffuse daylight. The cloud,
however, which shone with such extraordinary radiance under the
electric beam, was invisible in the ordinary light of the laboratory.

The quantity of mixed air and vapour within the experimental tube
could of course be regulated at pleasure. The rapidity of the action
diminished with the attenuation of the vapour. When, for example, the
mercurial colum