104 PHYSICAL SCIENCE 



energy of an isothermal system tends constantly 

 to decrease, enables us in many cases to evade 

 all molecular considerations, and to trace directly 

 the connections between various physical and 

 chemical phenomena. By this method it is 

 possible to -develop the theoretical relations of 

 many subjects without involving the molecular 

 hypothesis. Such treatment, using as its sole 

 principle of co-ordination the law of available 

 energy, ultimately rests on the experimental 

 impossibility of perpetual motion. 



This way of treating physical science was at 

 one time adopted by a certain number of chemists, 

 as a means of presenting their subject without 

 applying to it the language or conceptions of the 

 atomic theory, in terms of which even its simplest 

 experimental facts have come to be expressed. 

 In particular Franz Wald and Ostwald have 

 explained the phenomena of chemical combination 

 in definite proportions from the standpoint of 

 energetics. They have shown that the existence 

 of the two types known to us as elements and 

 compounds may be deduced from the thermo- 

 dynamic theory of equilibrium without reference 

 to atomic hypotheses. But, in the present state 

 of knowledge, such a doctrine seems limited in 

 its scope, and cases in which it ceases to be 

 sufficient will constantly recur in this volume. 

 For instance, the phenomena of highly rarified 

 gases have only been interpreted successfully by 

 the aid of strictly molecular conceptions. The 

 passage of electricity through gases, which will 

 be considered in a future chapter, again suggests 

 molecular hypotheses, and, in conjunction with 

 the phenomena of radio-activity, gives an extended 



