318 ON THE CONSERVATION OF FORCE. 



especially in the thorough conformity with law which 

 natural phenomena and natural products exhibit, and 

 in the comparative ease with which laws can be stated, 

 that this difference exists. Not that I wish by any means 

 to deny, that the mental life of individuals and peoples 

 is also in conformity with law, as is the object of philo- 

 sophical, philological, historical, moral, and social sciences 

 to establish. But in mental life, the influences are so 

 interwoven, that any definite sequence can but seldom 

 be demonstrated. In Nature the converse is the case. 

 It has been possible to discover the law of the origin 

 and progress of many enormously extended series of 

 natural phenomena with such accuracy and completeness 

 that we can predict their future occurrence with the 

 greatest certainty; or in cases in which we have power 

 over the conditions under which they occur, we can 

 direct them just according to our will. The greatest 

 of all instances of what the human mind can effect by 

 means of a well-recognised law of natural phenomena 

 is that afforded by modern astronomy. The one simple 

 law of gravitation regulates the motions of the heavenly 

 bodies not only of our own planetary system, but also of 

 the far more distant double stars ; from which, even the 

 ray of light, the quickest of all messengers, needs years 

 to reach our eye; and just on account of this simple 

 conformity with law, the motions of the bodies in ques- 

 tion, can be accurately predicted and determined both 

 for the past and for future years and centuries to a frac- 

 tion of a minute. 



On this exact conformity with law depends also the 

 certainty with which we know how to tame the impetuous 

 force of steam, and to make it the obedient servant of our 

 wants. On this conformity depends, moreover, the intel- 

 lectual fascination which chains the physicist to his sub- 

 jects. It is an interest of quite a different kind to that 



