IN THE LAST HALF-CENTUEY. 63 



they indicate definite physical concep- 

 tions, susceptible of mathematical treat- 

 ment, and giving rise to innumerable 

 deductions, the value of which can be 

 experimentally tested. The old notions 

 produced little more than floods of dia- 

 lectics ; the new are powerful aids to- 

 wards the increase of solid knowledge. 



Everyday observation shows that, of (2) Con- 



SGTVar 



the bodies which compose the material tion of 



energy. 



world, some are in motion and some are, 

 or appear to be, at rest. Of the bodies in 

 motion, some, like the sun and stars, ex- 

 hibit a constant movement, regular in 

 amount and direction, for which no ex- 

 ternal cause appears. Others, as stones 

 and smoke, seem also to move of them- 

 selves when external impediments are 

 taken away. But these appear to tend 

 to move in opposite directions : the bodies 

 we call heavy, such as stones, downwards, 

 and the bodies we call light, at least such 



