326 AIM AND PROGRESS OF PHYSICAL SCIENCE. 



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 different 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 generalisations from a 

 number of well-established special laws. We must endeavour 

 to eliminate the incidents of form and distribution in space which 

 masses under investigation may present by trying to find from 

 the phenomena attending large visible masses laws for the opera- 

 tion 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 poiii t of the mass is it especially clear that force is only 

 the law of action objectively expressed. The force arising from 

 the presence of such and such bodies is equivalent to the ac- 

 celeration 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 individual 

 points occurs. Its actual signification may be compared with 

 the facts and tested by them. The abstract conception of force 

 we thus introduce implies, moreover, that we did not discover 

 this law at random, that it is an essential law of phenomena. 



Our desire to comprehend natural phenomena, in other words 

 to ascertain their laws, thus takes another form of expression 

 that is, we have to seek out the forces which are the causes of 

 the phenomena. The conformity to Jaw in nature must be con- 



