70 FEAGMENTS OF SCIENCE. 



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 at- 

 tractions 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, because sufficient distance intervenes 

 between the attracting 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 crust consist for 

 the most part of substances whose atoms have already 

 closed in chemical union whose mutual attractions 

 are satisfied. Granite, for instance, is a widely dif- 

 fused substance; but granite consists, in great part, of 

 silicon, oxygen, potassium, calcium, and aluminum, 

 whose atoms united long ago, and are therefore dead. 

 Limestone is composed of carbon, oxygen, and a metal 

 called calcium, the atoms of which have already closed 

 in chemical union, and are therefore finally at rest. 

 In this way we might go over nearly the whole of the 

 materials of the earth's crust, and satisfy ourselves that 

 though they were sources of power in ages past, and 

 long before any creature appeared on the earth capable 

 of turning their power to account, they are sources of 

 power no longer. And here we might halt for a mo- 

 ment to remark on that tendency, so prevalent in the 

 world, to regard everything as made for human use. 

 Those who entertain this notion, hold, I think, an over- 

 weening opinion of their own importance in the sys- 



