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when however the temperature of such vapours is sufficiently 
increased, a change from spectra consisting of bands to spec- 
tra consisting of fine lines is to be looked for. This change 
has been observed in many cases. 
When a solid body is incandescent, the light emitted so 
as nearly to graze the surface may be considered due mainly 
to the surface molecules, but these being free on the side of 
the surface, but affected by other molecules on all other 
sides, the internal vibrations in these surface molecules will 
have a bias in a direction perpendicular to the surface. 
Thus the vibrations caused in the ether, which are propagated 
nearly grazing the surface, will preponderate in a direction 
perpendicular to the surface, or considering the vibrations in 
plane polarised light to be perpendicular to the plane of 
polarisation, the light emitted by such a body, so as to pass 
close to its surface, will be partially plane polarised, the 
plane of polarisation being parallel to the tangent plane to 
the surface of the body at the point of emission. 
In the case of an incandescent gas, the surface molecules 
are continually changed, and as a molecule may arrive at the 
surface in any position and is equally free on all sides, all 
trace of polarisation in this light will be destroyed. 
The fact that incandescent metallic plates do emit partially 
plane polarised light in directions nearly grazing the surface, 
the plane of polarisation being parallel to the surface, and 
that incandescent gases emit unpolarised light, has been 
observed by Arago. 
As the molecules at or near the surface of solids or liquids 
can cause vibrations in the ether, giving rise to emitted light, 
it is to be expected that, in some cases at least, it will be 
possible for light, if of sufficient intensity, when incident upon 
a body, to cause vibrations in the atoms constituting the 
molecules near the surface, but considering the difference of 
mass of the atoms of the body and of those of the ether, 
that the atoms of the body will vibrate slower than those of 
