48 



HABIT AND INTELLIGENCE. 



[chap. 



Gravita- 

 tion and 

 affinity. 



Energy 

 once be- 

 come 



actual has 

 been 

 parted 

 with. 



Miscon- 

 ceptions. 



Anecdote 

 of an iron- 

 master. 



pared to friction : the energy so lost, as in mechanical 

 friction, is converted into heat and dissipated, and cannot 

 be got back. 



The spontaneous combination of elements, in conse- 

 quence of their mutually attractive force, may be com- 

 pared to the fall of a weight by gravitation ; and the 

 separation of elements by electro-chemical decomposition, 

 in opposition to their mutual attraction, or affinity, may 

 be compared to the forcible lifting up of a weight. When 

 the weight falls, or when the elements combine, potential 

 energy becomes actual ; when the weight is lifted, or when 

 the compound is decomposed, actual energy becomes 

 potential. 



It is most important clearly to understand, that in all 

 cases of spontaneous combination the energy has been 

 parted with. In ordinary dynamics every one understands 

 this : every one is perfectly well aware that when the water 

 of a mill-stream has fallen so low that it can fall no lower, 

 it is impossible to get any more motive power out of it, 

 and motive power means useful energy. But in chemistry 

 it is not universally understood. Intelligent men still speak 

 of the power, or energy, locked uj) in every drop of water, 

 when they ought to speak of the energy that was given out 

 millions of years ago, when the water was formed by the 

 combination of oxygen and hydrogen ; and to speak of it 

 as energy, or motive power, is the same kind of error that 

 it would be to speak of the pressure of the ocean on 

 its bed as if any motive power were due to it. Water 

 is burnt hydrogen, and hydrogen, or any other com- 

 bustible, will not burn again after being burned. A story 

 is told, and is not at all too good to be true, of an iron- 

 master who had learned enough of chemistry to know that 

 water is composed of a combustible substance and a sup- 

 porter of combustion : he inferred that it must have the 

 properties of both its constituents, and tried the experiment 

 of blowing a furnace with steam instead of air, but of 

 course he only blew it out. 



When two moving bodies come into collision and bring 

 each other to rest, we say that their opposite momenta have 



