158 Science Religion and Reality 



development ot the phenomenon, is given instead decreasing values 

 to make it turn backwards, the whole system must re-traverse 

 exactly the steps which it has passed. Now Carnot, and, more 

 clearly still, Clausius, have shown that that does not happen at all in 

 the passage from thermal energy to kinetic energy. If we use a 

 given amount of work to raise the temperature of a body, we cannot 

 then return exactly to the initial stage of the process by inverting 

 the cycle. However perfect the machine may be, we shall never 

 obtain, by lowering the thermal level, the same quantity of work 

 with which we began. There will always remain a part of the 

 thermal energy which is not transformed into work, and which is 

 rendered unpotential. This irrevocability, this evolution of 

 Nature in a determined direction, is not explained by the mechanical 

 theory. If physical phenomena were due exclusively to the move- 

 ments of atoms, the mutual attractions of which depended only on 

 distance, they would have to be reversible. By inverting all the 

 initial velocities, the atoms, always subjected to the same forces, 

 ought to follow their trajectories in a contrary direction, in the 

 same way that the earth would describe in a retrograde direction 

 the same elliptical orbit that it describes in a forward direction, if 

 the original conditions of its movement were inverted. 



Carnot's principle shows, then, the impossibility of reducing 

 all the varied forms of energy to kinetic alone. Ostwald therefore 

 substituted for the atomic theory his own theory of energy, and 

 Duhem formulated a theory in which the various energies are 

 preserved as irreducible qualities. 



6. In the field of chemistry also new discoveries led scientists 

 to abandon the old idea of the indestructible atom and to substi- 

 tute for the atomic theory a theory of energy. The analyses made 

 by Crookes of the spectra of certain metals of the series of rare 

 earths, such as yttrium, samarium, and thorium, the discovery of 

 cathode rays, of X-rays, and of radio-active bodies, together with 

 the experimental demonstration that radio-activity does not belong 

 to certain bodies but constitutes a general property of matter, led 

 scientists to believe in an evolutionary genesis of the chemical 

 elements, according to which chemical species must be considered 

 as subject to a process of formation and of dissolution. 



Thus the historico-dynamic conception of reality received 

 fresh confirmation, and the possibility of different theories of 

 physical and chemical phenomena drew attention to the subject 



