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This caused me a momentary anxiety, for I did not know
how far, in describing my previous experiments, actions
might have been ascribed to pure cloudless vapor, which
were really due to those newly-observed nebulae. Inter-e
mittent discomfort, however, is the normal feeling of the
investigator; for it drives him to closer scrutiny, to greater
accuracy, and often, as a consequence, to new discovery.
It was soon found that the nebulw revealed by the beam
were also generated by the beam, and the observation
opened a new door into that. region inaccessible to sense,
which embraces so much of the intellectual life of the phys-
ical investigator.

What are those vapors of which we have been speak-
ing ? They are aggregates of molecules, or small masses of
matter, and every molecule is itself an aggregate of smaller
parts called atoms. A molecule of aqueous vapor, for ex-
ample, consists of two atoms of hydrogen and one of oxy-
gen. A molecule of ammonia consists of three atoms of '
hydrogen, and one of nitrogen, and so of other substances.
Thus the molecules, themselves inconceivably small, are
made up of distinct parts still smaller. When, therefore,
a compound vapor is spoken of, the corresponding mental
image is an aggregate of molecules separated from each
other, though still- exceedingly near, each of these being
compoSed of a group of atoms still nearer to each other.
So much for the matter which enters into our conception of
a vapor.1 T 0 this must now be added the idea of motion:
The molecules have motions of their own as wholes ,° their
constituent atoms have also motions of their own, which
are executed independently of those of the molecules 3 just

1 Newton seemed to consider that the molecules might be rendered
visible by microscopes; but of the atoms he appears to have entertained
a different Opinion. He finely remarks: “It seems impossible to see
the more secret and noble works of Nature Within the corpuscles, by
reason of their transparency.”-——(Herschel, 0% Light, Art. 1145.)

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