410 LAWS OF MOTION. 



be effected by a body which is impassive and 

 immotive. The reflection, therefore, from dif- 

 ferent surfaces, which the solar rays are known 

 to undergo, cannot be produced by any power 

 inherent in those surfaces, but must be referred 

 to the self-motive power of the rays alone, by 

 virtue of the elasticity they essentially contain ; 

 neither the solar rays, light, fire, nor air, 

 are indifferent to motion, or to rest ; they are 

 passive by the influence on them of an external 

 force, they are self-motive without it. 



2d. Neither is the assertion justified by the 

 fact, that these bodies can " persevere in a state 

 of motion in a right line." Such is the natural 

 existence of things in the world, that the matter 

 of which it is composed, has a constant ten- 

 dency to oppose resistance to the motion of a 

 body passing through it, and which resistance 

 alters the line of direction which the moving 

 body would otherwise pursue. The rays of 

 light are perpetually converged and diverged 

 from the influence of the medium through 

 which they pass, and seldom, if ever, move in a 

 right line. The same variation takes place in 

 bodies which are impelled to move by an exter- 

 nal force ; the influence of the medium through 

 which they pass, alters the direction of their 

 motion, from a right line to a curve; and on 

 the Newtonian hypothesis itself, the planets de- 

 scribe curves, and not right lines. 



3dly. To suppose that bodies persevere in a 



