14 



GALVANISM. 



to be hereafter mentioned, have been ob- 

 tained by magnetic galvanometers. 



5. Chemical changes effected by 

 Galvanism. 



(3 a.) In the Treatise on Electricity, 

 some of the chemical changes which re- 

 sult from the operation of this agent in 

 its ordinary form were noticed ; and ex- 

 periments were described in which water, 

 and a few'.saline bodies were decomposed 

 by a succession of electric discharges 

 from a powerful machine. But the 

 power of galvanism to effect changes in 

 the composition of bodies subjected to 

 its action is incomparably greater ; and 

 its application has led to a series of dis- 

 coveries which constitute a new era in 

 chemistry, and rank among the most 

 brilliant in the annals of physical 

 science. 



(40.) The chemical agency of galvanism, 

 unlike its power of eliciting heat, is 

 manifested, not while it is traversing 

 substances of great conducting powers, 

 but, on the contrary, when it meets with 

 impediments to its passage; and it is 

 exerted chiefly on substances, generally 

 fluids, which convey electricity only par- 

 tially and imperfectly. That we may 

 acquire clear ideas of the connection of 

 the chemical phenomena relating to gal- 

 vanism, it will be necessary to trace 

 them from their origin, and attend to 

 what takes place in the simplest gal- 

 vanic circle composed of two dissimilar 

 metals and an interposed fluid. 



(41.) If a plate of zinc, and another 

 of copper, be immersed in very dilute 

 sulphuric acid, - without touching or 

 communicating with each other, the 

 zinc will be acted upon by the acid ; 

 part of the water will be decomposed, 

 its oxygen combining with the zinc and 

 forming oxide of zinc ; and its hydrogen 

 will be disengaged in the form of gas 

 from the surface of the zinc plate. The 

 oxide of zinc, in proportion as it is pro- 

 duced, will be dissolved by the acid, 

 thereby forming sulphate of zinc. The 

 plate of copper, which has been immersed 

 in the same fluid, will, during all this 

 time, have undergone no change ; the 

 acid, in its diluted state, being incapable 

 of acting upon it. But if, while the 

 above process is going on, the metals be 

 brought into contact, either directly, or 

 by the intervention of some metallic in- 

 termedium, the following changes will 

 ensue. In the first place, the oxidation 

 and solution of the zinc will proceed 

 with much greater rapidity and energy 



than it did before ; and in the second 

 place, it will not be accompanied by the 

 evolution of the same quantity of hydro- 

 gen gas from the oxidating surface. 

 There will, indeed, be a disengagement 

 of hydrogen from the whole fluid, in 

 quantity exactly corresponding to that 

 of the oxygen derived from the water ; 

 but the greater part of this hydrogen 

 will now make its appearance on the 

 surface of the copper plate, whence it 

 will arise in a copious stream of bubbles. 

 But still the copper will itself remain 

 apparently unaffected by this change in 

 the circumstances of the experiment. 

 In process of time, indeed, when a con- 

 siderable proportion of sulphate of zinc 

 has been dissolved in the fluid, the 

 quantity of disengaged hydrogen is 

 found gradually to diminish, and a thin 

 film, composed partly of metallic zinc 

 and partly of filaments of oxide of zinc, 

 is deposited on the surface of the cop- 

 per ; as soon as this happens the gal- 

 vanic action ceases. 



(42.) If an acid, such as the nitric 

 acid, capable of acting upon the copper, 

 as well as upon the zinc, be employed 

 instead of the sulphuric acid, similar 

 phenomena will take place, with this 

 additional circumstance, that the action 

 of the acid upon the copper will cease 

 the instant the galvanic circuit is com- 

 pleted ; and instead of nitrous gas being 

 formed on the surface of the copper, 

 which happens before the circuit is 

 formed, only bubbles of pure hydrogen 

 will make their appearance ; and the 

 copper is protected from all further 

 action, the zinc being, as in the former 

 case, oxidated and dissolved with addi- 

 tional energy. It is on this principle 

 that Sir H. Davy has effected the pro- 

 tection of the copper sheeting of ships 

 from the corrosion of sea water, by 

 placing in contact with it pieces of zinc 

 or iron, on which sea water exerts a 



freater chemical action than on copper, 

 ee Phil. Trans, for 1824, p. 151, and 

 242; and for 1825, p. 328. 



(43.) In compound voltaic batteries, 

 the same chemical changes which have 

 been just described as occurring in the 

 simple galvanic circle, take place in 

 each of the portions of fluid intervening 

 in the compartments between the plates. 



(44.) The chemical agency of galva- 

 nism is exerted in a no less remarkable 

 manner on fluid conductors placed in the 

 circuit between the poles of the battery. 

 Among the simplest of its effects is the 

 resolution of water into its two gaseous 



