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2 l 8 FRAGMEN TS OF SCIENCE.

succession of impulses than that which produces the im-
pression of red. The vibrations of the" violet are about
twice as rapid as those of the red; in other words, the
range of the visible spectrum is about an octave.

There is no solution of continuity in this spectrum;
one color changes into another by insensible gradations.
It is as if an infinite number of tuning-forks, of gradually
augmenting pitch, were Vibrating at the same time. But
turning to another spectrum—that, namely, obtained from
the incandescent vapor of silver—you observe that it con-
sists of two narrow and intensely luminous green bands.
Here it is as if two forks only, of slightly different pitch,
were vibrating. The length of the waves which produce
this first band is such that 47,460 Of them, placed end to
end, would fill an inch. The waves which produce the
second band are a little shorter; it would take of these
47,920 to fill an inch. In the case of the first band, the
number of impulses imparted in one second to every
eye which now sees it, is 577 millions of millions; while
the number of impulses imparted in the same time by the
second band is 600 millions of millions. I now cast upon
the screen before you the beautiful stream of green light
from which these bands were derived. This luminous
stream is the incandescent vapor of silver. The rates of
vibration of the atoms of that vapor are as rigidly fixed as
those of two tuning-forks; and to whatever height the
temperature of the vapor may be raised, the rapidity of its
vibrations, and consequently its color, which wholly de-
pends upon that rapidity, remains unchanged.

The vapor of water, as well as the vapor of silver, has
its definite periods of vibration, and these are such as to
disqualify the vapor, when acting freely as such, from
being raised to a white heat. The oxyhydrogen flame, for
example, consists of hot aqueous vapor. It is scarcely
visible in the air of this room, and it would be still less

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