90 FRAGMENTS OF SCIENCE. 



beds of coal, composed chiefly of carbon, which has not yet 

 closed in chemical union with oxygen. Distance still inter- 

 venes between the atoms of carbon and those of oxygen, 

 across which the atoms may be impelled by their mutual 

 attractions, and we can do nothing more than utilize the 

 motion produced by this attraction. Once the carbon and 

 the oxygen have rushed together, so as to form carbonic 

 acid, their mutual attractions are satisfied, and, while they 

 continue in this condition, as dynamic agents they are dead. 

 A pound of coal produces by its combination with oxygen 

 an amount of heat which, if mechanically applied, would 

 raise a weight of 100 Ibs. to a height of twenty miles above 

 the earth's surface. Conversely, 100 Ibs. falling from a 

 height of twenty miles, and striking against the earth, 

 would generate an amount of heat equal to that devel- 

 oped by the combustion of a pound of coal. Wherever 

 work is done by heat, heat disappears. A gun which fires 

 a ball is less heated than one which fires blank cartridge. 

 The quantity of heat communicated to the boiler of a 

 working steam-engine is greater than that which could be 

 obtained from the recondensation of the steam after it had 

 done its work ; and the amount of work performed is the 

 exact equivalent of the amount of heat missing. We dig 

 annually nearly 100 millions of tons of coal from our pits. 

 The amount of mechanical force represented by this quantity 

 of coal seems perfectly fabulous. The combustion of a 

 single pound of coal, supposing it to take place in a minute, 

 would be equivalent to the work of 300 horses; and if we 

 suppose 120 millions of horses working day and night with 

 unimpaired strength, for a year, their united energies would 

 enable them to perform an amount of work just equivalent 

 to the heat to be derived from the annual produce of our 

 coal-fields. Our woods and forests are also sources of 

 mechanical energy, because they also have the power of 

 uniting with the atmospheric oxygen, and the molecular 



