ENERGY AND CHEMICAL CHANGE 



157 



But the production of energy in chemical change is not only thus 

 of practical importance; it is also of scientific interest, as will be 

 seen in the section on energy and chemical activity (see below). 



Interconvertibility of Forms of Energy: Conservation. 



At first sight, these different forms of energy seem to be quite 

 unrelated. But a relation between them can be found. If the 

 heat of a Bunsen flame or of the sun is brought under a hot-air 

 motor (Fig. 56) violent motion results. Again, if the motor 

 is connected with a dynamo, electricity may be generated. Still 

 again, if the current from the dynamo flows through an incandes- 

 cent lamp, heat and light are evolved. Conversely, when motion 

 of the hot-air motor is impeded by a 

 brake, heat appears. When a current 

 of electricity is run through the dynamo, 

 the armature of the latter turns and 

 motion results. But the most signifi- 

 cant facts are still to be mentioned. 

 The heat absorbed by the motor is 

 found to be greater when the machine is 

 permitted to move and do work, than 

 when it is not. Thus, it is found that 

 when work is done some heat disappears, 

 and this heat is, in fact, transformed 

 into work. Similarly, when the poles of 

 the dynamo are properly connected and 

 electricity is being produced, and only 



then, motion is used up. This is shown by the effort required 

 to turn the armature under these circumstances, and the ease 

 with which it is turned when the circuit is open. So, with a 

 conductor like the filament in the lamp, unless it offers resist 

 ance to the current and destroys a sufficient amount of electrical 

 energy, it gives out neither light nor heat. Finally, motion gives 

 no heat unless the brake is set, and effort is then demanded to 



FIG. 56 



