the l^undamental Problem of Nature, 3 



To determine (1) the constitution of the ultimate atoms and 

 molecules of matter, what they really are, and (2) their beha- 

 viour (the laws of their motions), are two great problems of Mo- 

 lecular Physics towards which all physical investigation is tend- 

 ing. These are the two important problems which first present 

 themselves for solution; but neither of them, as we shall see, is 

 the grand and fundamental problem. 



On a former occasion I referred to some considerations bear- 

 ing on the first of these two problems*. I shall now briefly 

 refer to the second, which, in consequence of its more immediate 

 relation to the ultimate question of scientific inquiry, is of much 

 more importance than the first. 



The second problem, we have seen, refers not to the nature of 

 the molecule, but to its motions. Now in regard to all physical 

 change or motion, no matter what the nature of that change or 

 motion may be, there are at the very outset two fundamental 

 questions which suggest themselves: — (1) What produces the 

 change — causes motion ? (2) What determines or directs it ? 



In regard to the first question, there is no diversity of opinion. 

 All agree that what produces change or causes motion is Force. 

 The second question, however, viz. what determines or directs the 

 motion, is not so easily answered. This question is not only the 

 more difficult of the two, but also by far the more important. 



All physicists agree that what is called Physical Law is just 

 the expression of the manner in which forces act in the produc- 

 tion of their effects, or '' the paths along which they travel to their 

 particular results,^^ as Mr. Lewes expresses itf. In the produc- 

 tion of all physical phenomena we have therefore two distinct 

 elements, viz. force, and the way or manner in which force acts 

 — force, and the paths along which it travels, so to speak — or, 

 in other words still. Force and the Laws of Force. 



One of the most important results of modern physical inquiry 

 has been to show that the various phenomena of Light, Heat, 

 Electricity, &c. are but different modifications in the action 

 of the same forces. When the forces take one path, we have 

 Light; taking another path, we have Heat; another produces 

 Electricity, and so on. Now it will be observed that the funda- 

 mental question is not, what is the particular force in action, or 

 upon what does its exertion depend, but rather what is it that 

 causes the force to act in the particular manner in which it 

 does act ? In other words, what determines the paths along 

 which it acts ? Physical phenomena are produced in general 

 by the motion of the molecules or of the atoms of bodies; 

 now the great question is not simply what produces the 



* Philosophical Magazine for December 1867. 



t Comte's Philosophy of the Sciences. By G. H. Lewes. Section V. 



B2 



