44 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
The old scholastic property of impenetrability, also, is one of 
the superficial notions of experience, gained in the same way as 
that of repulsion. It seems to pertain to solids—the typical mat- 
ter—with approximate accuracy, though calcined plaster of Paris 
and water, e. g., will occupy a good share of each other’s volume, 
and still form a highly porous solid. But a quart receiver full of 
hydrogen can have a quart of carbonic acid gas deftly introduced 
into it as into a void space; and so can a quart of water, at ordinary 
temperature and pressure, according to Gmelin, without increase of 
volume, although water is the type of material continuity. As to 
impenetrability in the molecule we can predicate nothing. The 
evolution of heat in chemical combinations indicates penetration of 
volume, with reorganization of the molecule in less space; and there 
is no reason, except a scholastic one, why two or more molecules, 
or even atoms, should not occupy the same place, as admitted by 
the highest authority—James Clerk-Maxwell. 
Dimension is also a common notion, derived similarly from supe- 
ficial and early experience. Solids alone have figure and assign- 
able dimension, though liquids have fixed volume, and gases variable 
volume, in inverse ratio to constraint; but even solids are of vary- 
ing and fluctuating dimensions, according to temperature, density, 
etc. Solidity and liquidity are, it is well known, but mere transi- 
tory conditions of material aggregation, for all matter is capable, 
by sufficient accession of molecular motion, of assuming that hyper- 
bolic or expansive condition which we call gaseous, and in this 
state dimension and impenetrability are meaningless terms. Con- 
cerning dimension as a necessary attribute of the unit of mass, 
Clerk-Maxwell says (Encyclopedia Britannica, 9th Ed., Vol. 3, p. 
37): “Many persons cannot get rid of the opinion that all matter 
is extended in length, breadth, and depth. This is a prejudice 
* * %* rising from our experience of bodies consisting of im- 
mense multitudes of atoms.’ That there is no necessary relation 
between mass and volume as there is, e. g., between mass and weight 
is shown to common experience by the notably different masses of 
a buck-shot and a pith-ball of the same dimensions, or of a cannon- 
ball and a child’s hydrogen balloon. A pellet of iridium equiva- 
lent in mass to the pith-ball might be microscopic, and, by extreme 
supposition, infinitesimal. We are not forced, however, to deny to 
