Energy. 319 



to another body because of another and a different way in which it af- 

 fects it. As there are a great many different sorts of motions which are 

 called work or molar motion, so there are many different sorts denom- 

 inated molecular motion; Blich as heat, light, chemism, electricity, mag- 

 netism, sound, gravitation. Any one of these possesses the potentiality 

 of becoming any other, directly or indirectly. That is, the same energy 

 ma}' appear in first one form and then another, till it makes the round 

 and appears in all. Some of these terms express phenomena or the ef- 

 fect which the motions have on us and not conditions of the motions 

 themselves, as will be seen further on. When energy appears by the 

 motion of one body, we know that the same amount of energy has 

 ceased to operate in some other body, or in some other form in the same 

 bod}^ ; and conversely when motion has ceased in one form we know 

 that it has gone on in some other form, or has done work which when 

 undone will } T ield again the same amount of energy. This is the princi- 

 ple called the conservation of energy, and it means that energy does not 

 start up from no-energy and cannot end in no-energ}\ Accordingly 

 then, molar and molecular motions are interchangeable with each other, 

 and when one ceases, some form of the other must begin. When the 

 motion of a falling body is arrested by the body striking the earth, the 

 full energy of the stroke is instantly transferred to the molecules of the 

 body itself and to the parts of the earth which were involved in the 

 stroke, these molecules being set to vibrating with more or less violence 

 according to the force of the blow ; such vibration being called heat. 

 Thus when a meteor falls through the air and strikes the earth it is al- 

 ways found to be hot. As long as it is above the atmosphere it is in- 

 visible because it is cold and solid mineral matter. As it passes through 

 the air its motion is retarded by the resistance of the air ; but this ap- 

 parently lost motion is not lost ; it is transferred to the particles of the 

 air and the particles of the meteor so that the vibrations set up in it, 

 which at first are heat, may become so rapid as to produce light. We 

 can then trace the track of the meteor through the air. But when 

 meteors do not become visible the} r nevertheless become hot in propor- 

 tion to that amount of their speed which is arrested, and when they 

 strike the ground and all the speed is stopped, the heat is increased 

 still more. So the whole of a molar movement may be converted into 

 heat. But on the other hand, while it is equally true that when mole- 

 cular vibration is arrested it gives rise to molar motion (or work), yet_ 

 as a common fact ( in human experience and observation ) the whole of 

 the vibrations in any given case never are arrested. A part of them 

 arc, while the rest are communicated to other bodies and continue as 

 molecular vibrations. Thus when heat is communicated to water, the 

 expansion of the steam furnishes work, but much heat is necessarily 



