Discontinuity in the Phenomena oj Radiation 47 



body loses energy of motion under these restricted conditions, it 

 gains energy of i^osition and vice-versa, or at all events this 

 conservation can be demonstrated for a system of bodies which, 

 while mutually acting and reacting on one another, are freed 

 from external influence. For many years, the view that heat 

 was a material substance, barred the natural extension of this 

 deduction to the view which regards the molecules and atoms of 

 a body as a system of bodies, themselves subject to dynamical 

 laws, and consequently brings their relative motions and positions 

 within the scope of the energy principle. With the gradual 

 abandonment of the belief in the material nature of heat, came 

 the conviction that heat was but the energy of molecular agitation. 

 This, at all events, was a tenable and highly plausible hypothesis, 

 and permitted a great extension of the energy principle on 

 the assumption that all intermolecular forces, such as cohesion 

 and those developed during friction or impact were subject to 

 Newton's laws. That was the state of affairs when in 1840 Joule 

 began his famous experiments on the equivalence between heat 

 developed by friction and the mechanical work expended in 

 maintaining the motion of the rubbing bodies. The positive 

 results of Joule's work and that of his successors constitute one of 

 the finest achievements of the 1 9th century. After that first step 

 other developments followed rapidly. Not only can matter in 

 its molecular and atomic form possess energy of motion ; it can 

 also possess potential energy. Energy of strain becomes resolved 

 into the mutual energy of position of the individual molecules 

 separated as they are from one another in their vibrational 

 movement against their mutual attractions. The absorption by 

 a liquid of latent heat as it is converted into vapour is but the 

 conversion of kinetic energy of the molecules of the fluid into 

 potential energy as those molecules escape from the liquid, and 

 not only rise against gravity but also separate considerably from 

 one another against the mutual attractions which hold them 

 together in the condensed form. The electric energy of the 

 materials of a voltaic cell, the chemical energies of various 



