PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 507 



react on the first. This is the previously mentioned principle of Newton 

 on the equality of action and reaction; so, for instance, a stone and the 

 earth attract each other with equal force; both fall towards each other; 

 the stone being a small mass does so visibly; the earth being - of greai 

 magnitude, (therefore moved in the most minute degree,) does so invisi- 

 bly; that is, the motion of the earth toward the stone is not sensible, 

 as it is of a non-measurable amount. The weight on a table exercises 

 the same pressure from above as the table from below. A spring pressed 

 by the hand exercises a counter-pressure in an equal degree, &c. Forces 

 then arc not self existing, but they are confined to matter, and manifest 

 themselves only by their actions. 



As forces are never observed as such, but can only be measured by their 

 actions, recently the question has repeatedly arisen whether the term 

 "force" ought not to be entirely banished from natural science. By 

 degrees an agreement has been arrived at in regard to this matter, so 

 that the actions of forces, which manifest themselves either as pressure 

 (tension) or as motion, are adequately termed potential or kinetic energy; 

 the latter term is susceptible of a mathematical expression. What form- 

 erly was called pressure or elasticity is now termed potential energy, 

 while the moving forces of old are now counted among the kinetic ener- 

 gies. The term "force" ought to be avoided in every case where its 

 action (work, capability of action, energy) is referred to; it may how- 

 ever h:- used in all those cases (and preserving the former usage of lan- 

 guage) where the cause of an effect, e. g. of a pressure or motion, is to 

 be indicated. 



On reviewing what has been said we find that although the older theo- 

 retical mechanics had certainly approached very near the proposition of 

 the conservation of energy, only by the discovery of the heat equiva- 

 lent and of the transformation of energy did it gain a precise definition, 

 as also a highly important general significance and a universal applica- 

 tion. At present, analytical mechanics has deduced this theorem mathe- 

 matically and made its application general. 



By the discovery of the heat equivalent, the theory of heat had to re- 

 linquish the hypothesis of a "caloric," transmitted by warmer bodies to 

 colder ones, which the latter absorbed for the purpose of raising their 

 temperature, and it had to turn to the hypothesis, according to which 

 radiant heat had its cause in oscillating aether atoms, and absorbed heat 

 in the effect of the oscillation of the molecules of bodies. The theories 

 of heat and of optics thereby obtained a common basis, namely that of 

 the oscillating aether atoms. In optics the theory of emission, accord- 

 ing to which exceedingly tine elastic and imponderable light-atoms were 

 radiated by luminous bodies with an enormous velocity, was discarded 

 earlier than the theory of caloric emission in the case of heat ; the cause of 

 this was a series of phenomena which could no longer be explained by 

 the emission hypothesis, but only by the transverse oscillations of that 

 extremely rare and highly elastic medium which we have termed a;ther. 



Let us examine in what manner the wave or undulatory theory ex- 



