AIM AND PKOGRESS OF PHYSICAL SCIENCE. 329 



combination or mixture, the mode of aggregation or molecular 

 structure that is to say, they may vary the mode of their 

 distribution in space. In their properties, on the other hand, 

 they are altogether unchangeable; in other words, when referred 

 to the same compound, as regards isolation, and to the same 

 state of aggregation, they invariably exhibit the same properties 

 as before. If, then, all elementary substances are unchangeable 

 in respect to their properties, and only changeable as regards 

 their combination and their states of aggregation that is, in 

 respect to their distribution in space it follows that all changes 

 in the world are changes in the local distribution of elementary 

 matter, and are eventually brought about through Motion. 



If, however, motion be the primordial change which lies at 

 the root of all the other changes occurring in the world, eveiy 

 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 merge 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 realisation 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 mole- 

 cules, while physiology has scarcely made a definite step in this 

 direction. 



This renders all the more important, therefore, a noteworthy 

 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 



