25 G KINETIC THEORIES OF GRAVITATION. 



expressing it in the language of the theories it opposes) I may call oethc- 

 rial nuclei. ... In defining atoms as centers of pressure, they are 

 thus no less distinguished on the one hand from centers of force than 

 from the little hard bodies of the ordinary theories; for such centers of 

 force are just as absolute and self-existent in the ordinary conception of 

 them as t hose little bodies ; and in a scientific theory there can (except as 

 temporary conveniences) be no absolute existences, entities. Hence 

 (mechanical) force, or the cause of motion, is conceived not as an entity 

 but as a condition, — the condition namely of a difference of pressure; 

 and the figure, size, and hardness of all bodies are conceived as relative, 

 dependent, and therefore changeable. There are thus no absolutely ulti- 

 mate bodies.''* In a later part of the same article (p. 3oC) the writer 

 objects to the ordinary atomic conception that "an hypothesis of infin- 

 itely hard atoms not merely requires in the consideration of the motion 

 of such an atom abstraction to be made of the interior relative motions, 

 also consequent on that difference of pressure which causes its external 

 relative motion, but explicitly denies any internal motion." The force 

 of the objection is not very obvious. There seems to be no more need 

 of conceiving internal motions in the ultimate unit of matter, for the pur- 

 poses of molecular physics, than there is for conceiving internal motions 

 in the planets for the purpose ef astronomical physics. 



In a brief summary of his views, in the succeeding volume, Mr. Gleu- 

 nie thus recapitulates: "Matter is conceived as made up, not of an 

 elastic aether and inelastic atoms, but of elastic molecules of different 

 orders us to size and density. If a rough physical conception of these 

 molecules be required, they may be conceived as setherial nuclei, the 

 sether of the nuclei of a lower being made up of nuclei of a higher or- 

 der, and so on ad infinitum? t 



It is somewhat difficult to criticise a scheme of gravitative pressure 

 so indeterminate in detail. The very function of a theory or hypothesis 

 is to formulate the unknown in terms of the known ; and in proportion 

 as a writer fails to do this, he fails to present us with an intelligible 

 theory or hypothesis. From the objection expressed, that "atoms " are 

 ordinarily conceived as self-existent bodies, while " in a scientific theory 

 there can be no absolute existences or entities," it might be supposed 

 that the author held the constituents of matter to be merely mathemat- 

 ical points (without dimension) forming the centers of repulsive 

 spheres, «* aetherial nuclei f but when we learn that these elastic mole- 

 cules are " of different orders as to mm and density," built up of success- 

 ive aggregates from the infinitely small, and that each exerts a repel- 

 hitit ' : pressure which is directly as the mass of its center," we are led 

 to conclude that these " rctherial nuclei" do possess a determinate mag- 

 nitude. If they are not to be considered as " absolute existeucesor enti- 

 ties," they are supposed to have at least sufficient substance to be moved 



* Phil. Mag , May, 1861, vol. xxi, pp. 351, 352. 

 t Phil. Mag., July, 1861, vol. xxii., p. 62. 



