178 SCIENCE AND THE HUMAN MIND 



which remains constant throughout all motion. In 

 the hands of the chemists, the balance showed that 

 this principle held good also when chemical changes 

 occurred. A body burning in air was not annihilated. 

 When the resultant gases were collected, the total 

 weight of body and air remained unaffected. 



And so it is in the case of energy. We find another 

 quantity besides mass which remains unchanged 

 throughout a series of transformations. Hence it is 

 convenient to recognize the existence of that quantity 

 and to give it a name. We call it energy, and measure 

 its changes by the amount of work done or by the 

 amount of heat developed. 



As we can neither create nor destroy matter, so we 

 can neither create nor destroy energy. We can but 

 convert matter or energy from one form into another. 



The importance of this result in the history of 

 physical science was immense. It gave a new method 

 of investigation, a new point of view from which 

 physical problems could be regarded. Since the final 

 energy of a closed system must be the same as that at 

 the beginning, it became possible in some cases to 

 predict the final state of the system without reference 

 to intermediate steps, to pass at once to the solution 

 of a problem without tracing the process by which the 

 goal was attained. As a practical guide in scientific in- 

 vestigation, the principle of the conservation of energy 

 is one of the great achievements of the human mind. 



But, if the principles of the conservation of matter 

 and energy hold good in all the circumstances we can 

 investigate, and to all the accuracy we can attain by 

 experiment, it was natural to stretch the principles 

 into the form of general laws. Matter became 



