CHAP, in.] ELECTRICITY AND MAGNETISM. 131 



frictional machines it is, as we have seen, a fair conductor. 

 Salt and saltpetre dissolved in water are good con- 

 ductors, and so are dilute acids, though strong sul- 

 phuric acid is a bad conductor. The resistance of the 

 liquid in the cells may be reduced, if desired, by using 

 larger plates of metal and putting them nearer together. 

 Gases are bad conductors : hence the bubbles of hydro- 

 gen gas which are given off at the copper plate during 

 the action of the cell, and which stick to the surface 

 of the copper plate, increase the internal resistance of 

 the cell by diminishing the effective surface of the plates. 



LESSON XIV. Chemical Actions in the Cell. 



159. The production of a current of electricity by a 

 voltaic cell is always accompanied by chemical actions 

 in the cell. One of the metals at least must be readily 

 oxidisable, and the liquid must be one capable of acting 

 on the metal. As a matter of fact, it is found that zinc 

 and the other metals which stand at the electropositive 

 end of the contact-series (see Art. 72) are oxidisable; 

 whilst the electronegative substances copper, silver, 

 gold, platinum, and graphite are less oxidisable, and 

 the last three resist the action of every single acid. 

 There is no proof that their electrical behaviour is due to 

 their chemical behaviour; nor is their chemical behaviour 

 due to their electrical. Probably both result from a yet 

 undiscovered cause. 



160. A piece of quite pure zinc when dipped alone 

 into dilute sulphuric acid is not attacked by the liquid. 

 But the ordinary commercial zinc is not pure, and when 

 plunged into dilute sulphuric acid dissolves away, a large 

 quantity of bubbles of hydrogen gas being given off from 

 the surface of the metal. Sulphuric acid is a complex 

 substance, in which every molecule is made up of a 

 group of atoms, 2 of Hydrogen, i of Sulphur, and 4 of 



