ther is more opaque than sulphuric; for a bright red heat
both are equally transparent; while, for a white heat, and still more
for a higher temperature, sulphuric aether is more opaque than formic.
This result gives us a clear view of the relationship of the two
substances to the luminiferous aether. As we introduce waves of
shorter period the sulphuric aether augments most rapidly in opacity;
that is to say, its accord with the shorter waves is greater than that
of the formic. Hence we may infer that the atoms of formic aether
oscillate, on the whole, more slowly than those of sulphuric aether.

When the source of heat is a Leslie's cube coated with lampblack and
filled with boiling water, the opacity of formic aether in comparison
with sulphuric is very decided. With this source also the positions of
chloroform and iodide of methyl are inverted. For a white-hot spiral,
the absorption of chloroform vapour being 10 per cent, that of iodide
of methyl is 16; with the blackened cube as source, the absorption by
chloroform is 22 per cent, while that by the iodide of methyl is only
19. This inversion is not the result of temperature merely; for when
a platinum wire, heated to the temperature of boiling water, is
employed as a source, the iodide continues to be the most powerful
absorber. All the experiments hitherto made go to prove that from
heated lampblack an emission takes place which synchronises in an
especial manner with chloroform. For the cube at 100' C, coated with
lampblack, the absorption by chloroform is more than three times that
by bisulphide of carbon; for the radiation from the most luminous
portion of a gas-flame the absorption by chloroform is also
considerably in excess of that by bisulphide of carbon; while, for the
flame of a Bunsen's burner, from which the incandescent carbon
particles are removed by the free admixture of air, the absorption by
bisulphide of carbon is nearly twice that by chloroform. _The removal
of the carbon particles more than doubles the relative transparency of
the chloroform_. Testing, moreover, the radiation from various parts
of the same flame, it was found that for the blue base of the flame
the bisulphide of carbon was most opaque, while for all other parts of
the flame the chloroform was most opaque. For the radiation from a
very small gas flame, consisting of a blue base and a small white tip,
the bisulphide was also most opaque, and its opacity very decidedly
exceed