radiation. The amount of aethereal disturbance, produced by the
combined atoms of a body, may be many thousand times that produced by
the same atoms when uncombined.

The pitch of a musical note depends upon the rapidity of its
vibrations, or, in other words, on the length of its waves. Now, the
pitch of a note answers to the colour of light. Taking a slice of
white light from the sun, or from an electric lamp, and causing the
light to pass through an arrangement of prisms, it is decomposed. We
have the effect obtained by Newton, who first unrolled the solar beam
into the splendours of the solar spectrum. At one end of this
spectrum we have red light, at the other, violet; and between those
extremes lie the other prismatic colours. As we advance along the
spectrum from the red to the violet, the pitch of the light--if I may
use the expression--heightens, the sensation of violet being produced
by a more rapid succession of impulses than that which produces the
impression 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 colour 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 vapour of silver--you observe that it consists
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 sees it, is 677
millions of millions; while the number of impulses imparted, in the
same time, by the second band is 600 millions of millions. We may
project upon a white screen the beautiful stream of green light from
which these bands were derived. This luminous stream is the
incandescent vapour of silver. The rates of vibration of the atoms of
that vapour are as rigidly fixed as those of two tuning-forks; and to
whatever height the temperature of the vapour may be raised, the
rapidity of its