84 Prof. Brlickc on Gravifalion and 



the particles of air in sound, and of the particles of retlicr in light, 

 without any essential difficulty : the material for this is at hand. 

 We are acquainted with masses great and small : we know their 

 motions ; it is only necessary, by an act of thought, to bring the 

 latter into the proper form, abstracting from their relations of 

 time and space, and all is in order. But it is otherwise with 

 forces. We know forces only as mental abstractions, as abstrac- 

 tions from the changes which our ego perpetually suffers from 

 the external world. We possess therefore no sensuous material 

 from which w^e can form an image of them. All attempts that 

 we make in this direction only end in our substituting in a some- 

 what offhand manner, things for forces which are utterly differ- 

 ent from them. We represent them by weights which pull and 

 springs which push, because in particular cases these produce 

 the same effects as the forces in question ; or we picture them as 

 lines, because the changes of the latter in magnitude and direc- 

 tion are suited to represent the corresponding changes of the 

 forces. It is simply because forces cannot be pictured before 

 the mind, that Newton found it impossible to form a conception 

 of an attractive force, inherent in bodies and capable of acting 

 through a vacuum on other bodies ; and he to whose glance the 

 heavens Avere more penetrable than his inner self, confounded 

 this impossibility with absurdity. Nothing, however, is further 

 removed from absurdity than the notion of an attractive force 

 acting at a distance. In the case of the abstractions of natural 

 science, the aphorism is perhaps truer than elsewhere, "By 

 their fruits ye shall know them." And as Faraday himself 

 remarks, Newton's law has been found valid "to an extent that 

 could hardly have been within the conception of New ton himself 

 when he gave utterance to the law." 



Its validity has thus been proved without our ever having 

 given up the notion of the actio in cUstans ; nay, the very idea of 

 an attractive force includes that of an action at a distance ; for 

 the only phsenomenon which immediately and necessarily leads 

 to the assumption of an attractive force, is the approximation of 

 two masses to each other without external impulse, and without 

 the action of other masses. As the bodies are supposed to ap- 

 proach each other, they could not of course have been already 

 in contact, and as the action of all other particles is excluded, the 

 force which causes them to approach must act across the vacuum 

 between both masses. We shall also, if I mistake not, find that 

 the law of gravitation, as it has been hitherto expressed, is by 

 no means in contradiction with the principle of the conservation 

 of force. I will here ask permission to proceed from Faraday's 

 own example. Let the mass A be separated from the mass B 

 by an external force; while this separation takes place, the 

 attraction diminishes, the attractive forces being in the inverse 



