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denly the beam darts through a luminous white cloud, which
has banished the preceding darkness. It has, in fact, shaken
asunder the molecules of the nitrite of amyl, and brought
down upon itself a shower of liquid particles which causes
it to flash forth in your presence like a solid luminous
spear. It is worth while to mark how this experiment
illustrates the fact that, however intense a luminous beam
may be, it remains invisible unless it has something to
shine upon. Space, though traversed by the rays. from all
suns and all stars, is itself unseen. Not even the ether
which fills space, and whose motions are the light of the
universe, is itself visible.

You notice that the end of the experimental tube most
distant from the lamp is free from cloud. Now the nitrite-
of-amyl vapor is there also, but it is unaffected by the
powerful beam passing through it. Let us make the trans-
mitted beam more concentrated by receiving it on a con-
cave silver mirror, and causing it to return by reflection into
the tube. It is still poWerless. Though a cone of light of
extraordinary intensity now traverses the vapor, no pre-
cipitation occurs, no trace of cloud is formed. Why ? Be-
cause the very small portion of the beam competent to de-
compose the vapor is quite exhausted by its Work in the
frontal portions of the tube. The great body of the light
which remains, after this sifting out of the few effectual
rays, has no“ power over the molecules of nitrite of amyl.
We have here, strikingly illustrated, what has been already
stated regarding the influence of period, as contrasted with
that of strength. For the portion of the beam which is here
ineffectual has probably more than a million times the ab-
solute energy of the effectual portion. It is energy specially
related to the atoms that we here need, which specially re-
lated energy being possessed by the feeble waves, invests
them with their extraordinary power. When the experi-
mental tube is reversed so as to bring the undecompcsed

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