AIM AND PKOGRESS OF PHYSICAL SCIENCE. 375 



If, however, motion be the primordial change which 

 lies at the root of all the other changes occurring in the 

 world, every elementary force is a force of motion, and the 

 ultimate aim of physical science must be to determine 

 the movements which are the real causes of all other 

 phenomena and discover the motive powers on which they 

 depend ; in other words, to merg'^ itself into mechanics. 



Though this is clearly the final consequence of the 

 qualitative and quantitative immutability of matter, it is 

 after all an ideal proposition, the realization of which is 

 still very remote. The field is a prescribed one, in which 

 we have succeeded in tracing back actually observed 

 changes to motions and forces of motion of a definite 

 kind. Besides astronomy, may be mentioned the purely 

 mechanical part of physics, then acoustics, optics, and 

 electricity ; in the science of heat and in chemistry, 

 strenuous endeavours are being made towards perfecting 

 definite views respecting the nature of the motion and 

 position of molecules, wliile physiology has scarcely made 

 a definite step in this direction. 



This renders all the more important, therefore, a note- 

 worthy advancement of the most general importance made 

 during the last quarter of a century in the direction we are 

 considering. If all elementary forces are forces of motion, 

 and all, consequently, of similar nature, they should all 

 be measurable by the same standard, that is, the standard 

 of the mechanical forces. And that this is actually the 

 fact is now regarded as proved. The law expressing this 

 is known under the name of the law of the Conservation 

 of Force. 



For a small group of natural phenomena it had already 

 been pronounced by Newton, then more definitely and in 

 more general terms by D. Bernouilli, and so continued of 

 recognised application in the greater part of the then 

 known purely mechanical processes. Certain amplifica- 



