852 Mr. J. S. Stuart Glennie on the 
19. I venture to offer this conception of atoms, not as a mere 
hypothesis, but as a fundamental scientific principle. For there 
is this involved in it—that as a phenomenon is scientifically ex- 
plained only when, and so far as, it is shown to be determined 
by other phenomena, the conception of Matter itself must be 
relative, and its parts be conceived as mutually determined. 
Now Pressure is not only an ultimate idea, including all those 
qualities of Matter classed by the metaphysicians as the Seeundo- 
primary, but is, unlike those, for instance, of Trinal extension, 
Ultimate ineompressibility, Mobility, and Situation (the primary 
qualities), not an absolute, but a relative conception, and, as 
such, that on which alone. can be founded a strictly scientific 
theory of material phenomena. For in the foundation of a 
theory based on the conception of the parts of matter as centres 
of pressure, there is nothing, properly speaking, hypothetical, 
as no absolute, intrinsic, or independent qualities of form, hard- 
ness, motion, &c. are postulated for atoms; and in their defini- 
tion nothing more is done than an expression given to our ulti- 
mate and necessarily relative conception of matter. 
In defining Atoms as Centres of Pressure, they are thus no less 
disting uished on the one hand from Centres of Force, than from 
the little hard bodies of the ordinary theories ; for such Centres 
of Force are just as absolute and self-existent in the ordinary 
conception of them as are those little bodies. And in a scientific 
theory there can, except as temporary conveniences, be no abso- 
lute existences,—entities. Ilence (Mechanical) Force, or the 
cause of motion, is conceived, not as an entity, but as a condi- 
tion—the condition, namely, of a difference of Pressure*; and 
the figure, size, and hardness of all bodies are conceived as rela- 
tive, dependent, and therefore changeable. There are thus no 
absolutely ultimate bodies. 
20. But the full justification of advancing this conception of 
Atoms as a fundamental scientific principle, is found in the prin- 
ciples of the modern critical school of philosophy—in that espe- 
cially of the relativity of knowledge. From such a point of view 
this principle cannot here be considered. I must limit myself, 
therefore, to a criticism of the opposed conception of atoms in a 
uniform ether, as developed by Prof. Challis, and to the attempt 
to show that, with the conception of atoms here offered, Prof. 
Maxwell’s somewhat arbitrary hypothesis of vortices becomes un- 
necessary. For I agree with the former in thinking that, “after 
all that can be done by this kind of research, an independent 
and @ priort theory ..... 1s stillneeded t ;” and I observe that 
* See the first part of this paper, Phil. Mag. April, p. 275. 
+ “On Theories of Magnetism and other ‘Forees, 1 in reply to Remarks 
by Professor Maxwell,” Phil. Mag. April, p. 253, 
