138 PROGRESS OF SCIENCE IN THE CENTURY, 



Transformations of Energy. Before methods of 

 measuring the different forms of what we call energy 

 had been elaborated, it was evident that one kind of 

 power was continually being changed into another. 

 Carbon and oxygen have in separation potential 

 energy the energy of chemical affinity for one an- 

 other, and this is manifested by the heat which they 

 give off when they unite; the heat may be in great 

 part utilised to convert water into steam ; the " expan- 

 sive force " of the steam lifts the piston ; the wheels 

 go round; the energy re-appears partly in the poten- 

 tial form of work done and partly in the heat which 

 results from overcoming friction. The energy of the 

 sunlight enables the plant to build up complex food- 

 stuffs out of simple raw materials ; substances of high 

 potential energy thus result; these become sources 

 of power to man and beast. The energy of chemical 

 separation may be transformed into heat, light, mag- 

 netism, electricity, and so on; or heat, light, and 

 electricity may be used to effect chemical separation. 

 Moreover, all the powers we can employ (except in 

 the case of tidal currents) are directly or indirectly 

 traceable to the energy radiated from the sun, or 

 to stores of potential energy in the earth, which again 

 we have to thank the sun for. 



Conservation. These considerations lead us to 

 the doctrine of the conservation of energy, which is 

 one of the foundations of Physics. It is an induc- 

 tion -from experience which states that " the total 

 amount of energy in a material system cannot be 

 varied, provided the system neither parts with 

 energy to other bodies nor receives it from them." 

 There may be degradation or dissipation of energy, as 



* Article " Energy," Chambers's Encyclopedia, by Dr. 

 W. Peddle. 



