CHEMICAL AFFINITY. 139 



acid, no chemical action takes place ; but mix the two acids, 

 and the immersed gold is chemically attacked and dissolved : 

 this is an ordinary chemical action, explained as the result of 

 a double chemical affinity. In hydrochloric acid, which is 

 composed of chlorine and hydrogen, the affinity of chlorine 

 for gold being less than its affinity for hydrogen, no change 

 takes place ; but when the nitric acid is added, this latter 

 containing a great quantity of oxygen in a state of feeble 

 combination, the affinity of oxygen for hydrogen opposes 

 that of hydrogen for chlorine, and then the affinity of the 

 latter for gold is enabled to act, the gold combines with the 

 chlorine, and chloride of gold remains in solution in the 

 liquid. Now, in order to exhibit this chemical force in the 

 form of electrical force, instead of mixing the liquids, place 

 them in separate vessels or compartments, but so that they 

 may be in contact, which may be effected by having a porous 

 material, such as unglazed porcelain, amianthus, &c., between 

 them. Immerse in each of these liquids a strip or wire of 

 gold : as long as the pieces of gold remain separated, no 

 chemical or electrical effect takes place ; but the instant they 

 are brought into metallic contact, either immediately or by 

 connecting each with the same metallic wire, chemical action 

 takes place- the gold in the hydrochloric acid is dissolved, 

 electrical action also takes place, the nitric acid is deoxidised 

 by the transferred hydrogen, and a current of electricity may 

 be detected in the metals, or connecting metal, by the appli- 

 cation of a galvanometer or any instrument appropriate for 

 detecting such effect. 



There are few, if any, chemical actions which cannot be 

 experimentally made to produce electricity : the oxidation of 

 metals, the burning of combustibles, the combination of oxy- 

 gen and hydrogen, &c., may all be made sources of electricity. 

 The common mode in which the electricity of the voltaic 

 battery is generated is by the chemical action of the oxygen of 

 water upon zinc ; this action is increased by adding certain 

 acids to the water, which enable it to act more powerfully upon 

 the zinc, or in some cases act themselves upon it ; and one 

 of the most powerful chemical actions known that of nitric 

 acid upon oxidable metals is that which produces the most 



