30 



HABIT AND INTELLIGENCE. 



[chap. 



Trans- 

 formation 

 of motion 

 into heat, 



and the 

 converse : 



Motion 

 into elec- 

 tricity, 



and the 

 converse 



Electricity 

 into heat. 



and the 

 converse 



Heat into 

 radiance, 

 and the 

 converse. 



Eadiance ; and shall proceed to enumerate their principal 

 modes of transformation. 



I. Motion is directly transformed into heat by collision 

 and friction, and also by the compression of air or any 

 other gas or vapour. 



Conversely, heat is transformed into motion by the 

 expansion of gases and vapours under pressure, as in the 

 steam-engine. It is experimentally proved that when any 

 gas or vapour expands into vacant space, so as to do 

 no work in expanding, its temperature is not altered by 

 expanding; but when it expands in such a way as to do 

 work by expanding (as steam does in the cylinder of a 

 steam-engine), its temperature falls ; and the heat that has 

 disappeared is the equivalent of the work done. 



II. Motion is transformed into electricity by the 

 common electric machine, and also by the magneto-electric 

 machine. 



Conversely, the energy of electricity is transformed into 

 that of motion, when a body is moved by electric 

 attraction or repulsion. 



III. Electricity is transformed into heat, when it is dis- 

 charged under such circumstances as to prevent work 

 from being done in any other way. This takes place in 

 the formation of the electric spark. 



Conversely, heat is transformed into electricity, and an 

 electric current is produced, when two substances that 

 conduct heat unequally are placed in contact, and heat 

 applied at the point of contact. Such currents are called 

 thermo-electric currents. 



IV. Heat is transformed into radiance by radiation. 

 And, conversely, radiance is transformed into heat by 



absorption. 



We have seen that by the law of the conservation of 

 energy, the energy that does work always reappears 

 as energy. This is self-evident when the resistance over- 

 come in doing the work consists in raising a weight ; the 

 energy reappears as the potential energy due to the raised 

 weight, and is ready to be converted into actual energy 

 again when the weight falls. When the resistance over- 



