the ground and mixing with the cold air above. The air thus ceases
to be a homogeneous medium, and every person must have observed the
effects of it upon objects seen through it which are very indistinctly
visible, and have a tremulous motion, as if they were "dancing in
the air." The very same effect is perceived when we look at objects
through spirits and water that are not perfectly mixed, or when we
view distant objects over a red hot poker or over a flame. In all
these cases the light suffers refraction in passing from a medium of
one density into a medium of a different density, and the refracted
rays are constantly changing their direction as the different currents
rise in succession. Analogous effects are produced when sound passes
through a mixed medium, whether it consists of two different mediums
or of one medium where portions of it have different densities. As
sound moves with different velocities through media of different
densities, the wave which produces the sound will be partly reflected
in passing from one medium to the other, and the direction of the
transmitted wave changed; and hence in passing through such media
different portions of the wave will reach the ear at different times,
and thus destroy the sharpness and distinctness of the sound. This
may be proved by many striking facts. If we put a bell in a receiver
containing a mixture of hydrogen gas and atmospheric air, the sound of
the bell can scarcely be heard. During a shower of rain or of snow,
noises are greatly deadened, and when sound is transmitted along an
iron wire or an iron pipe of sufficient length, we actually hear two
sounds, one transmitted more rapidly through the solid, and the other
more slowly through the air. The same property is well illustrated by
an elegant and easily repeated experiment of Chladni's. When sparkling
champagne is poured into a tall glass till it is half full, the glass
loses its power of ringing by a stroke upon its edge, and emits only
a disagreeable and a puffy sound. This effect will continue while the