276 Mr. J. S. Sttiart Glennie on the 
6. The special application of this principle is less to pheno- 
mena than to physical hypotheses. For as Force is thus 
conceived, not as an absolute entity acting upon matter, but 
as a condition of the parts of matter itself, “and as a condition 
determined by the relative masses and distances of these parts, 
any valid hypothesis of a force or of a motion to account 
for any set of phenomena is thus seen to imply an assertion 
as to relative masses and distances which can be more or less 
readily submitted to experiment or observation and analysis. 
And hypotheses of Forces which, like electric and magnetic 
fluids, or “‘ uniform elastic ethers, the sole source of physical 
power*,” exist absolutely, and are not merely expressions of facts 
of mass- and distance-difference, are by this principle rejected as 
unscientific. ‘ Lorsq’une branche de la Physique mathématique 
est ainsi parvenue a écarter tout principe douteux, toute hypo- 
thése restrictive, elle entre réellement dans une phase nouvelle. 
Kt cette phase parait définitive, car la série historique, et en 
méme temps rationnelle, des progrés accomplis, signale une ten- 
dance constante vers l’indépendance de toute loi préconguet.” 
How the mutually pressing or repelling parts of matter are to 
be conceived in order that from facts of difference in relative 
masses and distances alone, the forces of Molecular may be 
referred to similar conditions with those of Ordinary Mechanics, 
has been in the introductory paper indicated, and will in the 
second part of this paper be more fully developed. 
7. (II.) Motion, the effect of Force, whether mechanical, phy- 
sical, or chemical, may be distinguished as beginning or conti- 
nuous; and continuous motion as uniform or accelerated. The 
condition of the beginning of motion is a difference of pressure 
on the body that begins to move; the condition of a uniform 
continuous motion is a neutralization of the resisting pressure ; 
the condition of an accelerated continuous motion is a uniform 
or varying resisting pressure. 
8. This principle evidently embodies those of the Inertia of 
Matter, of the Composition of Motions, and of Accelerating 
Force. 
The principle of Inertia is the fundamental scientific principle 
of Non-spontaneity, or the impossibility of a motion undetermined 
by a change in the previously existing relations of the body. 
The inertia of a body or molecule is simply the relation between 
its pressure and that of the bodies acting upon it. All the 
meaning of this principle is in the relativity of the conception it 
gives of the phenomena of matter; it appears, therefore, to betray 
* Challis, “Ona Theory of Magnetic Force,” Phil. Mag. February 1861, 
p. 107. 
t Lamé, Théorie Analytique de la Chaleur, Discours préliminaire, p. Vi. 
