MATTER AND FORCE. 69 



In the fall of a cataract and the rush of the wind we 

 have examples of mechanical power. In the combina- 

 tions of chemistry and in the formation of crystals and 

 vegetables we have examples of molecular power. You 

 have learned how the atoms of oxygen and hydrogen rush 

 together to form water. I have not thought it necessary 

 to dwell upon the mighty mechanical energy of their act 

 of combination ; but it may be said, in passing, that the 

 clashing together of 1 Ib. of hydrogen and 8 Ibs. of oxygen 

 to form 9 Ibs. of aqueous vapour, is greater than the 

 shock of a weight of 1,000 tons falling from a height of 

 20 feet against the earth. Now, in order that the atoms 

 of oxygen and hydrogen should rise by their mutual 

 attractions to the velocity corresponding to this enor- 

 mous mechanical effect, a certain distance must exist 

 between the particles. It is in rushing over this that 

 the velocity is attained. 



This idea of distance between the attracting atoms 

 is of the highest importance in our conception of the 

 system of the world. For the matter of the world may 

 be classified under two distinct heads : atoms and mole- 

 cules which have already combined and thus satisfied 

 their mutual attractions, and atoms and molecules which 

 have not yet combined, and whose mutual attractions are, 

 therefore, unsatisfied. Now, as regards motive power, 

 we are entirely dependent on atoms and molecules of the 

 latter kind. Their attractions can produce motion, be- 

 cause sufficient distance intervenes between the attract- 

 ing atoms, and it is this atomic motion that we utilise in 

 our machines. Thus we can get power out of oxygen and 

 hydrogen by the act of their union ; but once they are 

 combined, and once the vibratory motion consequent on 

 their combination has been expended, no further power 

 can be got out of their mutual attraction. As dynamic 

 agents they are dead. The materials of the earth's 



