472 Mr. W. Crookes. On a Fourth State of Matter. [June 10, 



direction, by coercing them into a methodical rectilinear movement. 

 This I have shown to be the case in the phenomena which canse the 

 movements of the radiometer, and I have rendered such motion visible 

 in my later researches on the negative discharge 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 according to the extent to which this onward move- 

 ment has replaced the irregular motions which constitute the essence 

 of the gaseous condition, to that extent do I consider that the mole- 

 cules have assumed the condition 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 gaseons 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 path 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 

 continuous matter of which the fundamental properties — such as 

 pressure varying as the density, and so forth— are ascertained by ex- 

 periment. A gas is nothing- more than an assemblage 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 speculation. 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 senses 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 something retaining the properties of inertia and 

 weight would remain, matter, as we know it, would cease to exist. 

 Believe me, 



Dear Professor Stokes, 



Very sincerely yours, 



' WILLIAM CROOKES. 



