on the Mechanical Function of another. 417 



I have stated these axioms already, with some preliminary 

 explanation, in a communication to the Physical Society of 

 London (see for instance ' Nature/ vol.xlviii. pp. 62 and 101, 

 as well as Phil. Mag. July 1893) ; and I add another : — 



3. Material Particles never come into Contact, 

 meaning by material particles the actual substance of atoms 

 or molecules composing ponderable matter as ordinarily 

 known. It is for convenience that I use the term " material" 

 as opposed to " setherial : " I would not be understood to assert 

 or deny anything about the immateriality of aether — the 

 question would turn largely upon definition as well as upon 

 greater knowledge of properties than is at present possessed, — 

 but at least it is not material in the ordinary sense. 



For the acceptance of these three axioms I appeal to the 

 instinct of every physicist, based on his wide experience of 

 phenomena which it would be only tedious to recapitulate, 

 and I take them as constituting our real reason for postulating 

 the existence and constant activity of an aether ; they certainly 

 necessitate an immaterial connecting medium if action is ever 

 to occur. The ordinary argument for the existence of an 

 aether is obtained if in (2) the word " stress " is replaced by 

 the word waves, and in that form it is quite valid too, but force 

 is a simpler experience than light, and the undulatory theory 

 has only become established with difficulty and refinements of 

 observation. Something led Newton to postulate an aether, 

 and I apprehend it was really some form of the axiom here 

 numbered 2. Axiom 3 is not essential to the demand for the 

 existence of an aether, but it is essential to the aether's universal 

 activity. It may not be perfectly acceptable, especially by 

 those who seek to explain all actions in terms of material 

 collision — whether of ultra- hypothetical corpuscles or any 

 other kind of discontinuous substance. It is indeed intended 

 definitely to depreciate aud deny the probability of that 

 doctrine. 



Next I appeal to experience for support to the following 

 proposition : — premising that the word " mechanical " is here 

 throughout used to signify the behaviour of plain matter as 

 observed in bulk and thence inferentially extended to molecules, 

 but not so as to include the chemical behaviour of atoms. 

 The adjective is intended to exclude chemical and electric and 

 magnetic forces, it is not intended to discriminate molar from 

 molecular ; but in order that the term mechanical may apply 

 to molecules they must be dealt with individually, or else 

 statistically as in the kinetic theory of gases,, their motion is 

 not to be treated in the unorganized way appropriate to the 

 ideas of heat and temperature. 



