58 THE PHILOSOPHY OF BIOLOGY 



In all these friction is produced, and this friction passes 

 into heat. 



The potential chemical energy which results from 

 absorption of solar radiation by plants is principally 

 accumulated as coal. Apart from the interference of 

 man, this coal would slowly accumulate, perhaps it 

 would more slowly disappear by bacterial action, or by 

 physical transformations. In these transformations 

 the energy of the coal would become heat energy 

 and the potential energy of the gas produced by 

 bacterial activity. By man's agency the coal suffers 

 other transformations, and in the present phase of 

 civilisation it is his chief source of energy. It is 

 available for doing work of many kinds, and in all these 

 forms of work it becomes transformed by chemical 

 action (burning) into high temperature heat. 



We can cause this potential energy of coal to trans- 

 form into mechanical energy of machines, vehicles, 

 and ships in motion by causing it to pass into heat. 

 In the steam-engine, or gas-engine, a highly heated gas 

 (steam, or the mixture resulting from the explosion of 

 coal gas and air in the cylinder of the engine) expands 

 and propels a piston or rotates a turbine. (Obviously 

 in the petrol engine the same essential process takes 

 place.) We employ this kinetic energy directly in 

 transport, or we cause it to undergo other transfor- 

 mations. In the dynamo, kinetic energy of machinery 

 in motion transforms to electrical energy ; and this 

 may transform to radiant energy (light, heat in electric 

 radiators, wireless telegraphy radiations), or it may 

 transform to chemical energy (the manufacture of 

 carborundum in the electric furnace, for instance), 

 or it may transform again to the kinetic energy of 

 bodies in motion (electric traction). In innumerable 

 ways the human power of direction causes trans- 



