4.4, Mr. J.S. Stuart Glennie on the 
spirit of such remarks as these :— What we really want is not 
a variety of different methods of representing the forces, but the 
one true’physical signification of that which is rendered apparent 
to us by the phenomena, and the laws governing them. Of the 
two assuraptions most usually entertained at present, magnetic 
fluids and electric currents, one must be wrong, perhaps both ave 
. It is evident, therefore, that our phy sical views ave very 
doubtful ; ; and I think good would result from an endeavour to 
shake ourselves loose from such preconceptions as are contaimed 
in them, that we may contemplate for a time the force as much 
as possible in its purity*.” 
7. Now, in asystem of atoms as above defined, let the centres 
be of equal mass, and at equal distances; there will be no dif- 
ference of pressure on any one centre, no moving force will be de- 
veloped, and the conditions of equilibrium will be satistied. But 
it is clear that forces will be developed, or the general conditions 
of motion be fulfilled, either (a) by a difference in the masses of 
the centres, or (8) by a difference in the distances of the centres, 
in consequence of a displacement of any one of them, or (y), 
supposing a state of dynamic equilibrium established in the 
system, by its being brought in contact'with another system in 
a different state of such equilibrium. 
8. Consider more particularly the first of these conditions of 
the development of a force, and with the postulate that the pres- 
sures of atomic lines are directly proportional to the mass of the 
atomic centre, and inversely to the square of the distance there- 
from. But observe that this last property is conceived to arise, 
not from an absolute change in any individual line, whatever its 
distance from the centre, but because, from the lateral expansion 
of these lines, fewer will, when such expansion is unresisted, be 
cut at a greater than at a lesser distance by the same surface. 
In a system of atoms in which there is a difference in the 
masses of any one or more centres, and in which the law of the 
pressure of the atomic lines is as above, any two masses, whether 
unequal or equal, will tend to approach. 
And first, let the masses be unequal. 
Then the pressure of the lines from the one is greater than 
that of the lines from the other; consequently these lines will 
be mutually deflected, and hence the centres approach. 
Let the two masses be equal. 
Then, if all the masses of the system were equal, and all at 
the same distance from each other, their mutual repulsions would 
be equal in all directions, and they would remain at rest. But 
if, though two masses may be equal, either has, on the other 
side of it, a mass of a greater size, or at a,greater distance than 
* Faraday, ‘ Experimental Researches,’ vol. ii. p. 5380. 
