INTRODUCTION 5 



the chemical atoms, for long taken as ultimate 

 indivisible units. 



Further light has been thrown on these dark 

 places by the remarkable series of discoveries 

 through which M. and Mme. Curie and other 

 chemists gave us the radio-active elements 

 such as radium, and the parallel series in 

 which Rutherford and his fellow workers have 

 interpreted their properties as due to the dis- 

 integration of their atoms, as, one after another, 

 those atoms break down, and are transmuted 

 into other substances. 



Throughout these investigations we deal 

 with atomic and molecular conceptions in an 

 extreme form. We look even within the atom, 

 and examine its internal structure ; we trace the 

 electrons flying round the nucleus of the atom, 

 as we watch the planets swinging round the sun. 



It is remarkable that, in the other branch 

 of Physical Science, in which thermodynamic 

 principles are used, the methods chiefly employed 

 enabled us for a time to dispense altogether with 

 atomic and molecular theories. 



At the basis of the theory of physical and 

 chemical equilibrium lies Lord Kelvin's great 

 principle of the dissipation of energy. While 

 the total amount of energy in an isolated system 

 is unchanging and unchangeable, that energy is 

 tending always to become less available for the 

 performance of useful work. The availability of 

 the energy tends continually to become less. It 

 follows that permanent equilibrium can only be 

 attained when the limit has been reached and 

 the availability is a minimum. Such a theorem 

 is independent of molecular hypotheses ; in fact, 



