74 FRAGMENTS OF SCIENCE. 
The whole arrangement is shown in fig. 3, where L 
represents the electric lamp, s s' the experimental tube, pp 
the pipe leading to the air-pump, and F the test-tube con- 
taining the volatile liquid. The tube t t' 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 vapor 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 experiment 
is made in a darkened room, a tube which in ordinary day- 
light 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 vapor 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 
afterward whirled through the tube. 
The tube being again exhausted, the mixed air and vapor 
were allowed to enter it in the dark. The slightly conver- 
gent 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 con- 
tinued to act, showing 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 agam 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 vapor were allowed to enter the tube in diffuse day- 
light. The cloud, however, which shone with such 
extraordinary radiance under the electric beam, was in- 
visible in the ordinary light of the laboratory. 
The quantity of mixed air and vapor within the experi- 
