The Rev. J. H. Jellett on the Equilibrium and Motion of an Elastic Solid. 181 



General Classification of Bodies. 



I. — Hypothesis of Independent Action. 

 II. — Hypothesis of Modified Action. 



3. The classification which I propose here to adopt, and whicli forms the 

 basis of the present Memoir, is founded upon the following very obvious prin- 

 ciple. The force, or influence, which one particle or molecule exerts on 

 another, may show its effect either by causing a change in its state, or by 

 causing a change in its position. Either or both of these changes may affect 

 the influence which this particle in its turn exerts upon any of those around 

 it. Thus, for example, if m, m', ni", be three particles acting upon each other 

 by the ordinary attraction of gravitation, the action of m' upon m" will be mo- 

 dified by the action of m only so far as their distance from each other is 

 changed by it. The attraction of m has no power to change the attraction of 

 m' upon any other particle, except by altering its distance from that particle. 

 But the case would be altogether different if we supposed m, m', m", to be elec- 

 trified particles. In this case the action of m upon m' would modify the action 

 of that particle upon m", not only by changing the distance between them, but 

 also by changing their electrical state, and, therefore, the force which each ex- 

 erts upon the other. In the former case, if m' and m" maintain the same relative 

 position, the force which they mutually exert remains unchanged. In the se- 

 cond, even though the relative position of the two particles remains unaltered, 

 their mutual action will be modified by the presence of a third particle.* 

 From this distinction an obvious classification follows. In the first class we 

 place all bodies whose particles exert upon each other a force which is inde- 

 pendent of the surrounding particles; a force, therefore, which can be changed 

 only by a displacement of one or both of the particles under consideration. In 

 the second class, which includes all other bodies, the mutual action of two par- 

 ticles is supposed to be affected by that of the surrounding particles. 



* I do not, of course, mean to say, that in a case like that of electrified particles, change of state 

 in the particle itself may not be caused by change of position in the particles of some fluid which 

 pervades it. It is sufficient for my purpose, that in such a case the force which two particles 

 exert upon each other may be changed without a displacement of the particles themselves. 



