42 HABIT AND INTELLIGENCE. [cHAP. 



strong affinity for hydrogen, carbon, and iron, but hardly 

 any for nitrogen. 



The potential energy due to gravity becomes actual by 

 the falling of bodies, in which act it is transformed into 

 energy of motion. When the potential energy due to 

 capillarity becomes actual, it also is transformed into 

 energy of motion : this takes place when two drops of 

 mercury or of water rush together. But affinity differs 

 fundamentally from these two, in that its potential energy, 

 when it becomes actual, is not transformed into energy of 

 motion, but into heat or electricity. Combustion is not 

 only the most familiar but the best instance of the trans- 

 formation of chemical potential energy : the heat of com- 

 bustion is the transformed energy due to the affinity of 

 the oxygen and carbon that combine in the act of 

 combustion. But in many cases of combination it is 

 possible, by means of the arrangement of the voltaic 

 battery, to obtain it in the form of electricity instead 

 of heat. 



When the potential energy of gravity or capillarity 

 becomes actual, the change in the form of the energy does 

 not determine any change in the character of the bodies. 

 Water, for instance, does not change its character in 

 falling, nor do drops of water change their cliaracter as 

 water in rushing together by reason of their capillarity. 

 But when the potential energy of affinity becomes actual, 

 the transformation of the energy is always accompanied 

 by a change in the character of the substances ; they 

 combine, and the compound has properties unlike those 

 of either of the constituents. 

 Summary To sum up : Gravity acts at all distances and at every 



of their moment ; it is incapable of saturation, and is not elective. 



properties. ~ ,, 



Capillarity and affinity, on the contrary, act only at in- 

 sensible distances, and under favourable circumstances; 

 they are capable of saturation, and are elective. Gravity 

 and capillarity, when their potential energy becomes actual, 

 produce it as motion, and the character of the substances 

 remains unchanged. Affinity, on the contrary, produces 

 the energy in the form of either heat or electricity, and 



