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 we 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 ot 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 ihemselves 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 so?ne- 

 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. 



