THE ACTIVE FORCES OF LIVING ORGANISMS 9 



as their corporate existence lasts, all subjected to a 



common form of motion. This motion is the cause 



of or constitutes the rigidity which is the essence of 



the atomic form. Eigidity implies two things : it f;J^^^°*'°i" 



implies a certain degree of attractive power and a ^^°^ ^^\ 



■^ ° -^ essential 



certain degree of repulsive power. Chemistry teaches '0 rigidity. 

 us that these powers exist — that they are, in fact, the 

 basis of all the interchanges comprised under the 

 term ' chemical action.' Now, if we remember ' that 

 every substance in the universe attracts every other 

 substance with a force jointly proportional to the 

 mass of the attracting and of the attracted body, 

 and varying inversely as the square of the distance,'* 

 we pass at a single stride from the atomic theory -to 

 the law of gravity, from the atom to the world or 

 collection of atoms. 



Since, however, we have already come to the con- The con- 



' . ducting 



elusion that no conduction of force can take place medium in 



gravity. 



save through the medium of matter, we must suppose 

 that, when an apple falls to the ground, either force 

 has been transmitted to it from the earth through the 

 medium of the air or the ether, or that it has come 

 under the influence of force proceeding from the 

 ultra-mundane ether, and affecting it, the earth, and 

 all that appertains to it in common. But in the 

 attraction exerted by one heavenly body on another 

 the atmospheric air may be left out of account, and 

 the ether remains as the sole medium for the trans- 

 mission of attractive force. 



* BaUour Stewart, ' Physics,' p. 44, 



