SCIENCE. 
33 
varying directly with the temperature, increasing and dim- 
inishing in amplitude as the temperature rises and falls. 
The molecules in solids do not travel from one part to 
another, but possess adhesion and retain fixity of position 
about their centre of oscillation. Matter, as vve know it, has 
so high an absolute temperature that the movements of the 
molecules are large in comparison with their diameter, for 
the mass must be able to bear a reduction of temperature of 
nearly 300° C. before the amplitude of the molecular excur- 
sions would vanish. 
The state of solidity, therefore — the state which we are in 
the habit of considering par excellence as that of matter — is 
merely the effect on our senses of the motion of the discrete 
molecules among themselves. 
Solids exist of all consistencies, from the hardest metal, 
the most elastic crystal, down to thinnest jelly. A perfect 
solid would have no viscosity, i.e., when rendered discon- 
tinuous or divided by the forcible passage of a harder solid, 
it would not close up behind and again become continuous. 
In solid bodies the cohesion varies according to some un- 
known factor which we call chemical constitution; hence 
each kind of solid matter requires raising to a different tem- 
perature before the oscillating molecules lose their fixed 
position with reference to one another. At this point, vary- 
ing in different bodies through a very wide range of tem- 
perature, the solid becomes liquid. 
II. In liquids the force of cohesion is very much reduced, 
and the adhesion or the fixity of position of the centres of 
oscillating molecules is destroyed. When artificially 
heated, the inter-molecular movements increase in propor- 
tion as the temperature rises, until at last cohesion is broken 
down, and the molecules fly off into space with enormous 
velocities. 
Liquids possess the property of viscosity — that is to say, 
they offer a certain opposition to the passage of solid bodies; 
at the same time they cannot permanently resist such oppo- 
sition, however slight, if continuously applied. Liquids 
vary in consistency from the hard, brittle, apparently solid 
pitch to the lightest and most ethereal liquid capable of ex- 
isting at any particular temperature. 
The state of liquidity, therefore, is due to inter-molecular 
motions of a larger and more tumultuous character than those 
which characterize the solid slate. 
III. In gases the molecules fly about in every conceivable 
direction, with constant collision and enormous and con- 
stantly varying velocities, and their mean free path is suffi- 
ciently great to release them from the force of adhesion. 
Being free to move, the molecules exert pressure in all 
directions, and were it not for gravitation they would fly off 
into space. The gaseous state remains so long as the col- 
lisions continue to be almost infinite in number, and of in- 
conceivable irregularity. The state of gaseity, therefore, is 
pre-eminently a state dependent on collisions. A given 
space contains millions of millions of molecules in rapid 
movement in all directions, each molecule having millions 
of encounters in a second. In such a case the length of the 
mean free path of the molecules is exceeding small compared 
with the dimensions of the containing vessel, and the pro- 
perties which constitute the ordinary gaseous state of 
matter, which depend upon constant collisions, are ob- 
served. 
What, then, are these molecules ? Take a single lone 
molecule in space. Is it solid, liquid, or gas ? Solid it 
cannot be, because the idea of solidity involves certain pro- 
perties which are absent in the isolated molecule. In fact, 
an isolated molecule is an inconceivable entity, whether we 
try, like Newton, to visualise it as a little hard spherical 
body, or, with Boscovich and Faraday, to regard it as a 
centre of force, or accept Sir William Thomson’s vortex 
atom. But if the individual molecule is not solid, a fortiori 
it cannot be regarded as a liquid or gas, for these states are 
even more due to inter-molecular collisions than is the solid 
state. The individual molecules, therefore, must be classed 
by themselves in a disti ct state or category. 
The same reason applies to two or to any number of con- 
tiguous molecules, provided their motion is arrested or con- 
trolled, so that no collisions occur between them ; and even 
supposing this aggregation of isolated non-colliding mole- 
cules to be bodily transferred from one part of space to 
another, that kind of movement would not thereby cause 
this molecular collocation to assume the properties of gas ; 
a molecular wind may still be supposed to consist of isolated 
molecules, in the same way as the discharge from a mit- 
railleuse consists of isolated bullets. 
Matter in the fourth state is the ultimate result of gaseous 
expansion. By great rarefaction the free path of the mole- 
cules is made so long that the hits in a given time may be 
disregarded in comparison to the misses, in which case the 
average molecule is allowed to obey its own motion or laws 
without interference ; and if the mean free path is compatible 
with the dimensions of the containing vessel, the properties 
which constitute gaseity are reduced to a minimum, and the 
matter then becomes exalted to an ultra-gaseous state. 
But the same condition of things will be produced if by 
any means we can take a portion of gas, and by some ex- 
traneous force infuse order into the apparently disorderly 
jostling of the molecules in every direction, by coercing 
them into a methodical rectilinear movement. This I have 
shown to be the case in the phenomena which cause the 
movements of the radiometer, and I have rendered such 
motion visible in my later researches on the negative dis- 
charge in vacuum tubes. In the one case the heated lamp- 
black and in the other the electrically excited negative pole 
supplies the force majeure which entirely or partially 
changes into a rectilinear motion the irregular vibration in 
all directions ; and accordidg to the extent to which this on- 
ward movement has replaced the irregular motions which 
constitute the essence of the gaseous condition, to that ex- 
tent do I consider that the molecules have assumed the con- 
dition of radiant matter. 
. Between the third and the fourth states there is no sharp 
line of demarcation, any more than there is between the 
solid and liquid states, or the liquid and gaseous states ; 
they each merge insensibly one into the other. In the 
fourth state properties of matter which exist even in the 
third state are shown directly , whereas in the state of gas 
they are only shown indirectly, by viscosity and so forth. 
The ordinary laws of gases are a simplification of the 
effects arising from the properties of matter in the fourth 
state ; such a simplification is only permissible when the 
mean length of patch is small compared with the dimensions 
of the vessel. For simplicity’s sake we make abstraction of 
the individual molecules, and feign to our imagination con- 
tinuous matter, of which the fundamental properties — such 
as pressure varying as the density, and so forth — are ascer- 
tained by experiment. A gas is nothiug more than an 
assembly of molecules contemplated from a simplified point 
of view. When we deal with phenomena in which we are 
obliged to contemplate the molecules individually, we must 
not speak of the assemblage as gas. 
These considerations lead to another and curious specula- 
tion. The molecule — intangible, invisible, and hard to be 
conceived — is the only true matter, and that which we call 
matter is nothing more than the effect upon our sense of the 
movements of molecules, or, as John Stuart Mill expresses 
it, “a permanent possibility of sensation.” The space 
covered by the motion of molecules has no more right to be 
called matter than the air traversed by a rifle bullet can be 
called lead. From this point of view, then, matter is but a 
mode of motion ; at the absolute zero of temperature the 
inter-molecular movement would stop, and although some- 
thing retaining the properties of inertia and weight would 
remain, matter, as we know it, would cease to exist. 
NOTE BY THE DUKE OF ARGYLE. 
In the very interesting communication from Mr. Crookes 
on “ A Fourth State of Matter,” which is contained in 
Nature, vol. xxii. p. 153, there is a paragraph at the end 
which advances, as it seems to me, some most disputable 
propositions. 
Like many other questions of modern science, the ques- 
tion he raises is to a very large extent a question of defini- 
tion. But questions of definition are questions of the very 
highest importance in philosophy, and they need to be 
watched accordingly. 
