328 THE LIFE OF THE PLANT 



in this very knock all the energy acquired from the movement 

 of my hand. They do not move any longer, neither have they 

 any capacity for motion, i.e. tension. It seems evident that 

 energy has disappeared. But this is only apparent. The moment 

 the balls knocked against each other, the moment their motion 

 disappeared, there appeared another kind of energy heat. In 

 knocking, the balls became heated. It would be rather difficult 

 to prove this in the present case, because the rise in temperature 

 is only slight, but the fact cannot be doubted by any one who 

 has ever struck fire. Illustrations of this transformation of 

 energy are to be met with at every step. When metal is bored 

 the borings become very hot ; a piece of wood can be set on 

 fire by rubbing it against another piece of wood ; sparks fly 

 from under the brake of a train when it is suddenly stopped ; 

 a leaden bullet partly melts when it hits against a solid obstacle. 

 These phenomena of the transformation of mechanical energy 

 into heat long ago attracted attention ; they led the famous 

 Boyle, more than two centuries ago, to express an idea, which 

 has been scientifically developed only within the present genera- 

 tion : ' When we drive a big nail into a wooden plank/ writes 

 Boyle, ' we notice that it requires a great many blows before 

 it becomes heated ; but when we drive it down to its head, so 

 that it cannot move any longer, a few blows are sufficient to 

 make it hot. While every blow of the hammer drives the nail 

 deeper and deeper into the wood, a progressive movement of 

 its mass is provoked ; but the moment this movement is checked 

 the shock produced by the blow, unable any longer either to 

 drive the nail any further or split it, is necessarily bound to 

 spend itself on the inner oscillation of particles ; and heat as we 

 know consists of such motion.' Modern physics actually teaches 

 that heat is a rapid invisible but palpable oscillation of par- 

 ticles in a body. Thus the visible motion of balls produced 

 by the movement of my hand has passed into the invisible 

 motion of the particles of the balls. This motion, i.e. heat, 

 was communicated first to bodies in the nearest neighbourhood 

 of the balls ; then, spreading more and more, it is dispersed in 

 space. It is dispersed, but has not ceased to exist. The energy, 

 used by me to separate the balls, has not vanished entirely. In 

 doing this work I ultimately raised the temperature of the universe 

 though to an infinitely slight degree. Innumerable investiga- 



