THE PHILOSOPHICAL BASIS 37 



we recognise the conservation of momentum — a 

 name for the mathematical quantity obtained by 

 multiplying together the measures of mass and 

 velocity. Again, in reversible systems, where 

 physical or chemical changes may occur in either 

 direction with equal freedom, thermodynamics in- 

 dicates the conservation of another quantity, named 

 by Clausius, entropy. Momentum and entropy 

 are only conserved under restricted conditions ; in 

 physical systems the momentum of visible masses 

 is often destroyed, while In irreversible processes 

 entropy always tends to Increase. 



Mass and energy may seem to be conserved In 

 the conditions known to us, and we are justified 

 In extending the principle of their conservation to 

 all cases where those conditions apply. It does 

 not follow, however, that conditions unfamiliar 

 to us do not exist, in which mass and energy dis- 

 appear or come into existence. The persistence 

 of matter, for instance, might conceivably be an 

 apparent persistence. A wave, travelling over 

 the surface of the sea, seems to persist. It keeps 

 Its form unchanged, and the quantity of water In 

 it remains unaltered. We might talk about the 

 conservation of waves, and, perhaps, in so doing, 

 be as near the truth as when we talk of the per- 

 sistence of the ultimate particles of matter. But 

 the persistence of the wave Is an apparent phe- 

 nomenon. The form of the wave indeed truly 

 persists, but the matter in It is always changing — 

 changing in such a way that successive portions 

 of matter take, one after the other, an identical 

 form. Indications are not wanting that only in 

 some such sense as this is mass persistent. In a 

 later chapter we shall see that there is definite 



