1880.] Mr. W. Crookes. On a Fourth State of Matter. 471 



with constant collisions and enormons and constantly varying velocities, 

 and their mean free path is sufficiently 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 collisions continue to 

 be almost infinite in number, and of inconceivable irregularity. The 

 state of gaseity, therefore, is pre-eminently a state dependent on col- 

 lisions. 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 exceedingly small compared with the dimen- 

 sions of the containing vessel, and the properties which constitute the 

 ordinary gaseous state of matter, which depend upon constant colli- 

 sions, are observed. 



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 properties which are absent in 

 the isolated molecule. In fact, an isolated molecule is an incon- 

 ceivable entity, whether we try, like Newton, to visualize 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. Bat 

 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 distinct state or category. 



The same reasoning applies to two or to any number of contiguous 

 molecules, provided their motion is arrested or controlled, so that no 

 collisions occur between them ; and even supposing this aggregation 

 of isolated non- colliding molecules 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 mitrailleuse consists of 

 isolated bullets. 



Matter in the fourth state is the ultimate result of gaseous expan- 

 sion. By great rarefaction the free path of the molecules 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 motions or laws without interference ; and if the mean free 

 path is comparable to 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 extraneous force infuse 

 order into the apparently disorderly jostling of the molecules in every 



