AIM AND PROGRESS OF PHYSICAL SCIENCE. 371 



without exception^ and make this the test of its correct- 

 ness. If we can be assured that the conditions under 

 which the law operates have presented themselves, the 

 result must ensue without arbitrariness, without choice, 

 without our co-operation, and from the very necessity 

 which regulates the things of the external world as well 

 as our perception. The law then takes the form of an 

 objective power, and for that reason we call it force. 



For instance, we regard the law of refraction objectively 

 as a refractive force in transparent substances ; the law of 

 chemical affinity as the elective force exhibited by dif- 

 ferent bodies towards one another. In the same way, we 

 speak of electrical force of contact of metals, of a force 

 of adhesion, capillary force, and so on. Under these 

 names are stated objectively laws which for the most part 

 comprise small series of natural processes, the conditions 

 of which are somewhat involved. In science our con- 

 ceptions begin in this way. proceeding to generalizations 

 from a number of well-established special laws. We must 

 endeavour to eliminate the incidents of form and dis- 

 tribution in space which masses under investigation may 

 present by trying to find from the phenomena attending 

 large visible masses laws for the operation of infinitely 

 small particles ; or, expressed objectively, by resolving 

 the forces of composite masses into the forces of their 

 smallest elementary particles. But precisely in this, 

 the simplest form of expression of force — namely, of 

 mechanical force acting on a point of the mass — is it 

 especially clear that force is only the law of action ob- 

 jectively expressed. The force arising from the presence 

 of such and such bodies is equivalent to the acceleration 

 of the mass on which it operates multiplied by this mass. 

 The actual meaning of such an equation is that it ex- 

 presses the following law : if such and such masses are 

 present and no other, such and such acceleration of their 

 17 



