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changing its direction of motion upon the near approach of 
the molecule to another molecule, or to a containing vessel, 
which may be considered as equivalent to an impact. 
In an imperfect gas a molecule is supposed to be under the 
action of other molecules during a finite portion of the 
whole time of motion, this portion increasing as the gas 
approaches its state of saturation. 
Between the molecules of a body, and the atoms of a 
molecule, the luminiferous ether is supposed to exist. 
The vibrations in the ether which constitute radiant heat 
and light, are considered due to the vibrations of the atoms 
in the molecule, and not to the motion of the molecule as a 
whole ; the latter bearing some such relation to the ether, 
as a bell or a stretched string does to the air, the internal 
vibrations only in the two cases causing the vibrations in the 
surrounding media, which give rise respectively to light and 
sound. 
It appears obvious that as the motion of a molecule of a 
body as a whole increases, that is, as the temperature rises, 
the internal motion in the molecule also increases, considering 
that the action of one molecule upon another must be due to 
the mutual action of atoms, or to the interatomic forces, it 
seems probable that the internal vis viva in a molecule, to 
which the light is due, is proportional to the vis viva of the 
molecule as a whole, to which heat is to be referred. Thus, 
as the temperature of a body rises, the internal vis viva in 
the molecules increases, and the vis viva communicated to the 
ether also increases ; hence the intensity of the vibration in 
the ether increases, and at the same time the period of 
vibration diminishes, or waves of shorter length are con- 
tinually produced with increasing intensity. 
Hence as the temperature of a body rises radiant heat is 
given off, the intensity corresponding to a given wave length 
- constantly increasing, at last then vibrations in the ether, 
with wave lengths corresponding to the extreme red of the 
