15 



represent the weight of the smoke. The Court ladies laughed again ; 

 but clever Queen Bess nodded her head, and said that Sir Walter had 

 performed his promise. 



This is a simple instance of the remarkable truth which all reason- 

 ing and experience confirms, namely, that matter may be variously 

 transformed but cannot be destroyed. 



Now that which is true of matter is true in a still more remarkable 

 manner* of that which is called energy. The word is a technical one, 

 and I cannot very easily go into an accurate discussion and definition 

 of it ; but a very simple illustrative example may be given. Conceive 

 two equal inelastic masses, say masses of mud or putty, to be thrown so 

 as to meet each other with exactly equal velocities ; the result will be 

 that both will cease to move ; because each will convey to the other its 

 own energy, and as these energies are equal, neither can prevail over 

 the other, and the bodies will be brought to rest. If one of the bodies 

 were greater than the other, then the energy of this body would survive 

 in proportion to its excess over the other. 



It may be said, however, that in this case energy is destroyed, 

 inasmuch as before the collision there were two bodies in motion, 

 whereas after the collision the bodies were at rest, and so their energy 

 gone. This is true ; and the remark leads to the very pith of the 

 subject which I am now discussing. Regarding the question as one of 

 mere motion, it is sufficient to say that each body transfers its motion to 

 the other, and that the result is rest ; but in the process of collision and 

 transference of motion, something else happens : heat is generated : it is 

 a matter of familiar experience that to rub one substance against another, 

 or to knock one body against another produces heat : and, speaking 

 scientifically, when a body in motion is brought to rest by impact upon 

 another, its energy is converted into heat. Putting this result in a more 

 general form, we are led \o say that energy and heat are convertible, 

 that is, that energy produces heat, and that, conversely, heat may be 

 changed into energy. 



Of course I am now giving the merest hint of a grand compre- 

 hensive theory ; but you will, I think, easily perceive that the theory, of 



