26 
spectrum, will be caused with sufficient intensity to be 
visible, and thus the body will begin at first to glow with 
red light ; as the temperature still rises, and vibrations of 
shorter and shorter wave lengths become of visible intensity, 
the light emitted will gradually change from red to white. 
From Draper’s Law, that all bodies become incandescent 
simultaneously, as well as from other considerations, it seems 
probable that in all bodies the internal vis viva in the mole- 
cules bears the same ratio to the vis viva of the molecule as a 
whole. 
In solid and liquid bodies, the molecules being constantly 
under their mutual actions, and these actions being subject 
to constant change from the varying relative positions of the 
molecules, the atoms cannot assume any definite periods of 
vibration, but are constantly changing the time of vibration ; 
hence the vibrations in the ether will be constantly, and with 
extreme rapidity, changing their periods. This change 
having apparently no limit, and the effect upon the eye con- 
tinuing for a finite time, light of all wave lengths will appear 
to be given off simultaneously by such bodies when the tem- 
perature is sufficiently high ; in other words, incandescent 
solids and liquids will appear to give off Avhite light, which 
when analysed by a prism will yield a continuous spectrum. 
In the case of an incandescent Gas or Vapour sufficiently 
removed from a state of saturation to be considered per- 
fect, the atoms will be left to vibrate under the action 
of the interatomic forces only, and will thus assume periods 
of vibration all included in a certain set ; these vibra- 
tions Avill consequently cause vibrations in the ether corre- 
sponding only to certain definite wave lengths. Hence the 
spectra of such incandescent vapours will be broken, and 
consist only of a series of fine lines. 
With imperfect gases, or vapours not far removed from 
their points of saturation, the intermediate phenomenon 
of spectra broken, but consisting of bands, is to be expected : 
