226 



SCIENCE. 



[N. S. Vol. III. No. 59. 



which could transmit the vibrations, and 

 thus the hypothetical ether took its place 

 in the scientific mind. The phenomena of 

 polarization require that the vibrations 

 shall be transverse, and therefore the 

 ether must be a solid. The calculations of 

 Lord Kelvin have shown that a medium 

 with properties, such as must be ascribed 

 to the ether to account for the facts known, 

 would not be stable, in other words, that it 

 could not exist. Probably in order to save 

 the electro-magnetic theory from a like 

 fate, the immortal Herz, to whom this 

 theory owes so much, expressly declines to 

 see anything in it but a sj^stem of six dif- 

 ferential equations. 



The task of science is to find the relations 

 , that exist between realities, measurable 

 quantities, so that when some are known 

 others can be deduced. This idea is not 

 new. Mayer, fifty-three years ago, discov- 

 ered the equivalence of the natural forces, 

 or, as we say to-day, of the different forms 

 of energy. Then Clausius, Helmholtz and 

 "W. Thompson thought it necessary to in- 

 terpret the law of the equivalence of the 

 different forms of energy by assuming that 

 all the different forms of energy are funda- 

 mentally the same, that is to say, mechanical 

 energy. This was distinctly a backward 

 step. 



How is it then possible, by means of 

 such an abstract idea as energy, to form a 

 conception of the universe, which in clear- 

 ness can be compared with the mechanical ? 

 "What do we then know of the physical 

 world? Plainly only that which comes to 

 us through our sensory organs. What 

 conditions must be satisfied in order that 

 one of these organs shall be affected ? We 

 may turn the matter in any way we 

 please, we find no common feature but this: 

 The sensory organs are affected by energy 

 changes between them and their environ- 

 ment. In a world, the temperature of 

 which is everywhere the same as that of 



our bodies, we could not know anything of 

 heat, just as we do not feel the constant 

 atmospheric pressure under which we live. 

 Only when we produce spaces with other 

 pressures do we gain any knowledge of it. 



It is often said energy is imaginary, while 

 matter is the reality! The author answers : 

 On the contrary, matter is a product of the 

 imagination, that we have constructed very 

 imperfectly in order to represent the per- 

 manent in the everlasting changes. 



According to the author, matter and en- 

 ergy are not to be thought of as distinct, as 

 for example, body and soul. If we attempt 

 to think of matter as separate from the 

 various forms of energy nothing is left. 

 Matter is, in fact, nothing but a group of 

 different energies arranged in space. He 

 then makes use of this crude illustration. 

 Imagine yourself struck with a cane. 

 What do you feel, the cane or its energy? 

 Of course, it is the energy. The cane at 

 rest is harmless. 



Everything that has hitherto been repre- 

 sented by the aid of the conceptions of force 

 and matter, and much more, can be repre- 

 sented by means of the conception of en- 

 ergy. We make a great gain by indulging 

 in no hypotheses in regard to the connec- 

 tion between the different forms of energy 

 except that which is specified in the law of 

 conservation, and we gain the freedom of 

 studying the different phenomena object- 

 tively. 



Finally, it may be asked, is energy the 

 last reality ? However, necessary and use- 

 ful for the understanding of nature energy 

 may be, is there nothing beyond it? Or 

 are there phenomena which cannot be fully 

 expressed by the now known law of energy ? 

 The author expresses the belief that energy 

 is not suflScient to enable us to deal with all 

 nature. It will probably appear in the fti- 

 ture as a special case of still more general 

 relations of the form of which we have at 

 present no conception. 



