82 The Outline of Science 



eclipse of the sun, is that light is subject to gravitation. A ray 

 of light from a star is bent out of its straight path when it passes 

 near the mass of the sun. Professor Eddington tells us that we 

 have as much right to speak of a pound of light as of a pound of 

 sugar. Professor Eddington even calculates that the earth re- 

 ceives 160 tons of light from the sun every year! 



ENERGY: HOW ALL LIFE DEPENDS ON IT 



As we have seen in an earlier chapter, one of the funda- 

 mental entities of the universe is matter. A second, not less im- 

 portant, is called energy. Energy is indispensable if the world 

 is to continue to exist, since all phenomena, including life, depend 

 on it. Just as it is humanly impossible to create or to destroy a 

 particle of matter, so is it impossible to create or to destroy 

 energy. This statement will be more readily understood when 

 we have considered what energy is. 



Energy, like matter, is indestructible, and just as matter 

 exists in various forms so does energy. And we may add, just as 

 we are ignorant of what the negative and positive particles of elec- 

 tricity which constitute matter really are, so we are ignorant of 

 the true nature of energy. At the same time, energy is not so 

 completely mysterious as it once was. It is another of nature's 

 mysteries which the advance of modern science has in some 

 measure unveiled. It was only during the nineteenth century 

 that energy came to be known as something as distinct and 

 permanent as matter itself. 



Forms of Energy 



The existence of various forms of energy had been known, 

 of course, for ages; there was the energy of a falling stone, the 

 energy produced by burning wood or coal or any other substance, 

 but the essential identity of all these forms of energy had not been 

 suspected. The conception of energy as something which, like 



