284 The Outline of Science 



we have seen on a previous page, are in a state of continual mo- 

 tion, and the more vigorous the motion the hotter the body. As 

 wood or coal burns, the invisible molecules of these substances are 

 violently agitated, and give rise to ether waves which our senses 

 interpret as light and heat. In this constant movement of the 

 molecules, then, we have a manifestation of the energy of motion 

 and of heat. 



That energy which disappears in one form reappears in 

 another has been found to be universally true. It was Joule who, 

 by churning water, first showed that a measurable quantity of 

 mechanical energy could be transformed into a measurable 

 quantity of heat energy. By causing an apparatus to stir water 

 vigorously, that apparatus being driven by falling weights or a 

 rotating flywheel or by any other mechanical means, the water 

 became heated. A certain amount of mechanical energy had been 

 used up and a certain amount of heat had appeared. The re- 

 lation between these two things was found to be invariable. 

 Every physical change in nature involves a transformation of 

 energy, but the total quantity of energy in the universe remains 

 unaltered. This is the great doctrine of the Conservation of 

 Energy. 



13 



Substitutes for Coal 



Consider the source of nearly all the energy which is used 

 in modern civilisation coal. The great forests of the Carbonifer- 

 ous epoch now exists as beds of coal. By the burning of coal 

 a chemical transformation the heat energy is produced on which 

 at present our whole civilisation depends. Whence is the energy 

 locked up in the coal derived? From the sun. For millions of 

 years the energy of the sun's rays had gone to form the vast vege- 

 tation of the Carboniferous era and had been transformed, by 

 various subtle processes, into the potential energy that slumbers 

 in those immense fossilized forests. 



