of Attractive and Repulsive Forces. 187 



that consequently beyond a certain limit of magnitude the 

 attractive action at the boundary of the spherical space may 

 exceed the repulsive action. In that case the atoms of the 

 space may be conceived to be divided into component groups, 

 the atoms of each of which are mutually repulsive as far as 

 depends on the dynamical effect of their proper waves ; and it 

 is evident that under these circumstances the groups will also 

 be mutually repulsive. Those groups situated near the centre 

 of the spherical space will be kept in equilibrium in great 

 measure by their mutual repulsions ; but in proportion as a 

 group is further from the centre it will be acted upon in the 

 direction from the centre by a residual repulsion which re- 

 quires to be counteracted by attractive action towards the 

 centre. The latter force is supplied by the waves composed 

 of the individual emanations from the whole number of atoms, 

 for which waves kn will be a very large number. The least 

 number of atoms of a given substance which constitute a mass 

 which is thus self-contained may be called a molecule of that 

 substance. It has long appeared to me that the received 

 mode of attributing attractive and repulsive forces to occult 

 virtues emanating from particles or points leaves wholly un- 

 explained why some forces are repulsive and others attractive. 

 This fact is fully explained by the present theory, which ac- 

 counts for such diverse action of forces by modes of pressure of 

 the aetherial medium. 



21. I do not propose to discuss here the effect of the inci- 

 dence of a series of waves on a movable sphere. This is the 

 case of Example VII. in the ' Principles of Physics/ a solu- 

 tion of which to small quantities of the first order is given 

 in pages 296-298. Another solution, including terms of the 

 second order, which is attempted in pages 44(>-452 of the 

 same work, I subsequently found to be faulty on account of 

 its involving the supposition of uncompounded plane-waves. 

 I believe, however, that the conclusion arrived at in p. 452, 

 namely that the accelerative action on a movable sphere has 

 a constant ratio to that on a fixed sphere, admits of being 

 satisfactorily established. 



22. The atomic and molecular forces to which the foregoing 

 theory applies are usually said, on experimental grounds, to 

 have sensible effects only at insensible distances from their 

 centres of emanation. This description is in agreement with 

 the present theory, as will appear from the following conside- 

 rations. In the expression obtained in art. 18 for the whole 

 pressure on a single atom resulting from the incidence upon 

 it of the waves from all the atoms of a given spherical group, 

 we may suppose the value of m, at any distance R from the 



