THE LAW OF SUBSTANCE. 235 



of the persistence of force, illustrates the constant 

 reciprocity of the two chief types of substance, ether 

 and mass. 



The great law of nature, which, under the title of 

 the " law of substance," we put at the head of all 

 physical considerations, was conceived as the law of 

 " the persistence of force " by Robert Meyer, who 

 first formulated it, and Helmholtz, who continued the 

 work. Another German scientist, Friedrich Mohr, of 

 Bonn, had clearly outlined it in its main features ten 

 years earlier (1837). The old idea of force was, after 

 a time, differentiated by modern physics from that of 

 energy, which was at first synonymous with it. Hence 

 the law is now usually called the "law of the persis- 

 tence of energy." However, this finer distinction 

 need not enter into the general consideration, to 

 which I must confine myself here, and into the 

 question of the great principle of the " persistence of 

 substance." The interested reader will find a very 

 clear treatment of the question in Tyndall's excellent 

 paper on " The Fundamental Law of Nature," in his 

 Fragments of Science. It fully explains the broad 

 significance of this profound cosmic law, and points 

 out its application to the main problems of very 

 different branches of science. We shall confine our 

 attention to the important fact that the " principle of 

 energy " and the correlative idea of the unity of 

 natural forces, on the basis of a common origin, are 

 now accepted by all competent physicists, and are 

 regarded as the greatest advance of physics in the 

 nineteenth century. We now know that heat, sound, 

 light, chemical action, electricity, and magnetism are 

 all modes of motion. We can, by a certain apparatus, 

 convert any one of these forces into another, and 



