MATTER AND ENERGY 65 



resins. Wood tar is an ingredient of lubricating oils and axle 

 greases. Acetic acid is used in the preparation of certain 

 dyes and medicines, and in the manufacture of vinegar. In the 

 old-fashioned method of charcoal making, these products were 

 lost; in the modern method they are collected and used. 



Matter and energy. When wood is burned in the air, a 

 small pile of ashes is left, and we think of the bulk of the wood 

 as destroyed. It is true that we have less matter that is avail- 

 able for use or that is visible to sight, but, nevertheless, no 

 matter has been destroyed. The matter of which the wood 

 is composed has merely changed its character; some of it is 

 in the condition of ashes, and some of it in the condition of 

 invisible gases, such as carbon dioxide, but none of it has 

 been destroyed. It is a principle of science that matter can 

 neither be destroyed nor created ; it can only be changed, or 

 transformed, and it is our business to see that we do not heed- 

 lessly transform it into substances which are valueless to us and 

 our descendants ; as, for example, when our magnificent forests 

 are recklessly wasted. The smoke, gases, and ashes left in 

 the path of a raging forest fire are no compensation to us for 

 the valuable timber destroyed. The sum total of matter has 

 not been changed, but the amount of matter which man can 

 use has been greatly lessened. 



The principle just stated forms one of the fundamental laws 

 of science, called the law of the conservation of matter. 



A similar law holds for energy. We can transform electric 

 energy into the motion of trolley cars, or we can make use of 

 the energy of streams to turn the wheels of our mills, but in 

 all these cases we are transforming, not creating, energy. 



When a ball is fired from a rifle, most of the energy of the 

 gunpowder is utilized in motion, but some is dissipated in pro- 

 ducing a flash and a report, and in heat. The energy of the 

 gunpowder has been scattered, but the sum of the various 



