OF MOLECULAR FORCES. 99 



tion of the jxirticles are repulsive, and vary according to an inverse 

 power of the distance greater than 2. 



III. It is lastly assumed, that the ether exists (or at least is capable 

 of existing) as one mass held together by the attraction of its elementary 

 molecules. This assumption is necessary, in order that the dispersion of 

 the medium which would naturally result from the repulsive forces which 

 regulate the vibration of its particles, may be thereby prevented. 



The result which is derived from this necessary assumption is, that 

 each particle exerts (in addition to the repulsive force before mentioned) 

 an attractive force, which varies according to Newton's Law of universal 

 gravitation. 



By reversing the problem, I have been able to shew, that though 

 Newton's law is the only one which cannot enable the particles to vibrate, 

 yet it is the only law of force which can enable them to constitute and 

 maintain themselves a permanent medium, without endangering, or in any 

 way affecting their vibrating or luminiferous property. 



I have on these grounds not hesitated to express my opinion, that 

 the particles of the luminiferous ether are each endued with two forces of 

 distinct characters and uses; one attractive, to preserve themselves a per- 

 manent medium, varying inversely as the square of the distance; and the 

 other repulsive, to which is due their luminiferous property, varying in a 

 higher inverse ratio of the distance than the square. 



A SYSTEM OF DETACHED PARTICLES. 



1. If V denote the sum of the quotients formed by dividing each 

 attracting body by its distance from the attracted body ; then V = C is 

 the equation of a surface at any point of which if the attracted body 

 be placed, it will begin to move in the direction of a normal. 



N2 



