136 PROGRESS OF SCIENCE IN THE CENTURY. 



a part of the shower which would otherwise batter 

 upon it . . It is necessary also to suppose that par- 

 ticles and masses of matter have a cage-like form, so 

 that enormously more corpuscles pass through them 

 than impinge upon them ; else the gravitation action 

 between two bodies could not be as the product of 

 their masses." * But this speculation is only a pro- 

 visional stop-gap. 



To the easy-going materialists, if any survive, the 

 ignoramus of one of our leading physicists should 

 give pause. "Directly we use the term 'weight,' 

 we are confronted with the fact that not yet have we 

 any real clew to that astonishing fact of universal 

 gravitation." f 



SUMMARY. The foundation of modern physics is 

 in Newton's Principia (1687) whose value is more 

 fully appreciated at the end than it was at the begin- 

 ning of the nineteenth century. 



CONSERVATION OF ENERGY. 



The Idea of Energy. Energy is a convenient term 

 &>r the power of doing work which is possessed by a 

 material system, or by the ether which modern phys- 

 ics has invented as a hazy background of matter. A 

 stream flowing down a valley illustrates energy of 

 motion, it may turn mill-wheels or bear away bridges ; 

 the reservoir on the plateau illustrates energy of posi- 

 tion, which intention or accident may at any moment 

 bring into operation. These two types of power are, 

 as every one knows, called kinetic energy and poten- 

 tial energy. Whether the kinetic energy be expressed 

 in visible motion, as of the stream, or invisible mo- 



P. O. Talt, Recent Advancei in Phytical Science, 1876, 

 pp. 299-300. 



tProf. Oliver J. Lodge, "Modern Views of Matter," 

 Internal. Monthly, I. (1900), p. 526. 



