THE MULTIPLICATION OP EFFECTS. 443 



forces that widely diverge. In the case lately cited, of a 

 body shattered by violent collision, besides the change of the 

 homogeneous mass into a heterogeneous group of scattered 

 fragments, there is a change of the homogeneous mo- 

 mentum into a group of momenta, heterogeneous in both 

 amounts and directions. Similarly with the forces we know 

 as light and heat. After the dispersion of these by a radiat- 

 ing body towards all points, they are re-dispersed towards all 

 points by the bodies on which they fall. Of the Sun's rays, 

 issuing from him on every side, some few strike the Moon. 

 These being reflected at all angles from the Moon's sur- 

 face, some few of them strike the Earth. By a like 

 process the few which reach the Earth are again diffused, 

 through surrounding space. And on each occasion, such 

 portions of the rays as are absorbed instead of reflected, 

 undergo refractions that equally destroy their parallel- 

 ism. More than this is true. By conflict with 

 matter, a uniform force is in part changed into forces 

 differing in their directions; and in part it is changed into 

 forces differing in their kinds. When one body is struck 

 against another, that which we usually regard as the effect, 

 is a change of position or motion in one or both bodies. But 

 a moment's thought shows that this is a very incomplete 

 view of the matter. Besides the visible mechanical result, 

 sound is produced; or, to speak accurately, a vibration in 

 one or both bodies, and in the surrounding air: and under 

 some circumstances we call this the effect. Moreover, the 

 air has not simply been made to vibrate, but has had currents 

 raised in it by the transit of the bodies. Further, if there is 

 not that great structural change which we call fracture, 

 there is a disarrangement of the particles of the two bodies 

 around their point of collision ; amounting in some cases to 

 a visible condensation. Yet more, this condensation is ac- 

 companied by disengagement of heat. In some cases a 

 spark — that is, light — results, from the incandescence of a 

 portion struck off ; and occasionally this incandescence is as- 



