24 
A paper was read entitled “ An Attempt to refer some 
Phenomena attending the Emission of Light to Mechanical 
Principles,” by R. B. Clifton, M.A., Professor of Natural 
Philosophy in Owens College. 
The author attempted to show, by analogical arguments, 
that it is possible to give some account of certain phenomena 
attending the Emission of Light, by assuming principles closely 
resembling, if not identical with, those adopted by Professor 
Clausius in his well known paper on “ The Nature of the 
Motion which we call Heat.”* 
Matter is assumed in all cases to have its atoms grouped 
together into molecules, an assumption which seems necessary 
when the different allotropic states of certain substances are 
considered. 
These molecules are assumed to be in motion, and also the 
atoms to be vibrating in the molecules ; for, whatever may 
be the laws of the forces which bind together the atoms in 
the molecules, it is impossible to imagine the molecules to be 
in motion, and to be subject to mutual actions, without 
causing motion of the component atoms. 
In solids and liquids the molecules are supposed to remain 
continually within the spheres of action of neighbouring 
molecules, so that the internal motion in a molecule is never 
due to the undisturbed action of the interatomic forces — the 
only difference between solids and liquids being that in the 
former the same molecules are constantly neighbours, while 
in the latter a molecule may completely change its place in 
the liquid, and also that in liquids a molecule may perform 
complete rotations round axes through its centre of gravity, 
while in solids this is not generally possible. 
In a perfect gas a molecule is supposed to be under the 
action of other molecules, only for a portion of time in- 
definitely small with respect to the whole time of motion, 
and its centre of gravity describes a polygonal path, only 
* Phil. Mag., vol. sir., for 1857. 
