294 ROUGH WAYS MADE SMOOTH. 



any error arise from the use of the ordinary method of 

 expression, so long as we carefully hold in remembrance 

 that it is only employed for convenience, and must not be 

 regarded as scientifically precise. 



Electricity may be excited, as I have said, in many 

 ways. With the ordinary electrical machine it is excited by 

 the friction of a glass disc or cylinder against suitable 

 rubbers of leather and silk. The galvanic battery developes 

 electricity by the chemical action of acid solutions on metal 

 plates. We may speak of the electricity generated by a 

 machine as friction al electricity, and of that generated by a 

 galvanic battery as voltaic electricity ; in reality, however, 

 these are not different kinds of electricity, but one and the 

 same property developed in different ways. The same also 

 is the case with magnetic electricity, of which I shall 

 presently have much to say : it is electricity produced by 

 means of magnets, but is in no respect different from fric- 

 tional or voltaic electricity. Of course, however, it will be 

 understood that for special purposes one method of pro- 

 ducing electricity may be more advantageously used than 

 another. Just as heat produced by burning coal is more 

 convenient for a number of purposes than heat produced by 

 burning wood, though there is no scientific distinction 

 between coal-produced heat and wood-produced heat, so 

 for some purposes voltaic electricity is more convenient than 

 frictional electricity, though there is no real distinction 

 between them. 



Every one knows that when by means of an ordinary 

 electrical machine electricity has been generated in sufficient 

 quantity and under suitable conditions to prevent its dis- 

 persion, a spark of intense brilliancy and greater or less 

 length, according to the amount of electricity thus collected, 

 can be obtained when some body, not similarly electrified, 

 is brought near to what is called the conductor of the 

 machine. The old-fashioned explanation, still repeated in 

 many of our books, ran somewhat as follows : 'The 

 positive electricity of the conductor decomposes the neutral 



