494 THE CONSERVATION OF ENERGY 



the musical part alone being competent to act upon our thermometers 

 or to aifect our nerves. 



" In this case, then, the heat not only imparts actual energy to the 

 vibrating atoms, but also accomplishes what we may call interior 

 ivork ; it performs work within the body heated, by forcing its 

 particles to take up new positions. When the body cools, the forces 

 which were overcome in the process of heating come into play ; the 

 heat which was consumed in the recession of the atoms being restored 

 upon their approach." 



These extracts will render the idea of conservation sufficiently 

 plain. It may be explained that heat is communicated to a body either 

 by the impulses of the ethereal waves, which is termed radiation, or 

 by contact with the heating body, which is termed conduction ; both 

 these operations are combined in most cases of transference of heat. 



Let us now consider the action of heating the lead from the point 

 of view of the supporters of action at a distance. 



One simple mode of conceiving the motion in this case is to suppose 

 the particles in pairs, each pair revolving about its centre of gravity. 



Since equal increments of heat produce equal increments of ex- 

 pansion within certain limits, the cohesive attraction between the 

 particles of a pair must be constant between these limits, just as in 

 raising a mass against the constant force of gravity the height to 

 which it is raised is proportional to the energy expended. Again, 

 while the temperature remains constant, the velocities will be con- 

 stant. Hence the orbits will be circles. Since the increments of 

 temperature and expansion are constant for equal increments of heat, 

 they must bear a fixed relation to one another. Hence, as heat is 

 applied, the increment of the radius must bear a fixed relation to 

 the increment of the square of the transverse velocity corresponding 

 to it. Thus, as the heat increases, the particles must be whipped 

 around, as, it were, in gradually widening spirals by the impulses of 

 the ethereal waves, or of the particles of the heating body, or both ; 

 and when the temperature becomes stationary, the impulses must be 

 so nicely adjusted as just tO' give the particles the velocity due to 

 the circle in which they revolve at the final temperature, under the 

 influence of the constant attraction. When one considers also that 

 the planes of the revolving particles must be lying in every conceiv- 

 able direction (since the dilatation is equal in all directions), which 

 adds immensely to the chances of collision and consequent destruction 



