MATTER AND FOKCE. 71 



the molecules of the earth's crust vast in relation to 

 us, but trivial in comparison to the total store of which 

 they are the residue still remain. They constitute 

 our main sources of motive power. By far the most 

 important of these are our beds of coal. Distance still 

 intervenes between the atoms of carbon and those of 

 atmospheric oxygen, across which the atoms may be 

 urged by their mutual attractions ; and we can utilise 

 the motion thus produced. 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. Our woods and forests are also sources of 

 mechanical energy, because they have the power of 

 uniting with the atmospheric oxygen. Passing from 

 plants to animals, we find that the source of motive 

 power just referred to is also the source of muscular 

 power. A horse can perform work, and so can a man ; 

 but this work is at bottom the molecular work of the 

 transmuted food and the oxygen of the air. We inhale 

 this vital gas, and bring it into sufficiently close 

 proximity with the carbon and the hydrogen of the 

 body. These unite in obedience to their mutual at- 

 tractions ; and their motion towards each other, pro- 

 perly turned to account by the wonderful mechanism 

 of the body, becomes muscular motion. 



One fundamental thought pervades all these state- 

 ments : there is one tap root from which they all spring. 

 This is the ancient maxim that out of nothing nothing 

 comes; that neither in the organic world nor in the 

 inorganic is power produced without the expenditure of 

 power ; that neither in the plant nor in the animal 

 is there a creation of force or motion. Trees grow, 

 and so do men and horses; and here we have new 

 power incessantly introduced upon the earth. But its 



