NATURE 717 



complex; all we can say of it is but approximate, and our formulae 

 are constantly requiring new terms. 



But the ranks are unbroken, the relations that we have discovered 

 between objects we thought simple still hold good between the same 

 objects when their complexity is recognized, and that alone is the 

 important thing. Our equations become, it is true, more and more 

 complicated, so as to embrace more closely the complexity of nature; 

 but nothing is changed in the relations which enable these equations 

 to be derived from each other. In a word, the form of these equa- 

 tions persists. Take for instance the laws of reflection. Fresnel estab- 

 lished them by a simple and attractive theory which experiment 

 seemed to confirm. Subsequently, more accurate researches have 

 shown that this verification was but approximate; traces of elliptic 

 polarization were detected everywhere. But it is owing to the first ap- 

 proximation that the cause of these anomalies was found in the exist- 

 ence of a transition layer, and all the essentials of Fresnel's theory 

 have remained. We cannot help reflecting that all these relations 

 would never have been noted if there had been doubt in the first 

 place as to the complexity of the objects they connect. Long ago it 

 was said: If Tycho had had instruments ten times as precise, we 

 would never have had a Kepler, or a Newton, or Astronomy. It is 

 a misfortune for a science to be born too late, when the means of 

 observation have become too perfect. That is what is happening at 

 this moment with respect to physical chemistry; the founders are 

 hampered in their general grasp by third and fourth decimal places; 

 happily they are men of robust faith. As we get to know the proper- 

 ties of matter better we see that continuity reigns. From the work 

 of Andrews and Van der Waals, we see how the transition from the 

 liquid to the gaseous state is made, and that it is not abrupt. Sim- 

 ilarly, there is no gap between the liquid and solid states, and in the 

 proceedings of a recent Congress we see memoirs on the rigidity of 

 liquids side by side with papers on the flow of solids. 



With this tendency there is no doubt a loss of simplicity. Such and 

 such an effect was represented by straight lines ; it is now necessary to 

 connect these lines by more or less complicated curves. On the other 

 hand, unity is gained. Separate categories quieted but did not sat- 

 isfy the mind. 



Finally, a new domain, that of chemistry, has been invaded by 

 the method of physics, and we see the birth of physical chemistry. It 

 is still quite young, but already it has enabled us to connect such 

 phenomena as electrolysis, osmosis, and the movements of ions. 



From this cursory exposition what can we conclude? Taking all 

 things into account, we have approached the realization of unity. 

 This has not been done as quickly as was hoped fifty years ago, and 



