254 THE PRINCIPLES OF SCIENCE. 



rally to depend upon the molecule ; thus, the power of 

 certain substances to cause the plane of polarization of a 

 ray of light to rotate, is exactly the same whatever be its 

 degree of density, or the diluteness of the solution in 

 which it is contained. Taken as a whole, the physical 

 properties of substances and their quantitative laws, pre 

 sent a problem of infinite complexity, and centuries must 

 elapse before any moderately complete generalizations on 

 the subject become possible. 



Uniform Properties of all Matter. 



Some laws are held to be true of all matter in the 

 universe absolutely, without exception, no instance to the 

 contrary having ever been noticed. This is the case with 

 the laws of motion, as laid down by Galileo and Newton. 

 It is also conspicuously true of the law of universal 

 gravitation. The rise of modern physical science may 

 perhaps be considered as beginning at the time when 

 Galileo showed, in opposition to the Aristotelians, that 

 matter is equally affected by gravity, irrespective of its 

 form,^ magnitude, or texture. All objects fall with equal 

 rapidity, when disturbing causes, such as the resistance of 

 the air, are removed or allowed for. That which was 

 rudely demonstrated by Galileo from the leaning tower of 

 Pisa, was proved by Newton to a high degree of approxi 

 mation, in an experiment which has already been referred 

 to (vol. ii. p. 55). 



^ Newton formed two pendulums of as nearly as possible 

 similar outward shape, by taking two equal round wooden 

 boxes, and suspending them by equal threads, eleven 

 feet long. The motion of each pendulum was therefore 

 equally subject to the resistance of the air. He filled one 

 box with wood, and in the centre of oscillation of the 



