prior to the Introduction of the Potentials. 53 



One of the investigations with which Cavendish occupied 

 himself was a comparison between the conducting powers of 

 different materials for electrostatic discharges. The question 

 had been first raised by Beccaria, who had shown* in 1753 that 

 when the circuit through which a discharge is passed contains 

 tubes of water, the shock is more powerful when the cross-section 

 of the tubes is increased. Cavendish went into the matter 

 much more thoroughly, and was able, in a memoir presented to 

 the Eoyal Society in 1775,f to say : " It appears from some 

 experiments, of which I propose shortly to lay an account before 

 this Society, that iron wire conducts about 400 million times 

 better than rain or distilled water that is, the electricity meets 

 with no more resistance in passing through a piece of iron wire 

 400,000,000 inches long than through a column of water of the 

 same diameter only one inch long. Sea- water, or a solution of 

 one part of salt in 30 of water, conducts 100 times, or a saturated 

 solution of sea-salt about 720 times, better than rain-water." 



The promised account of the experiments was published in 

 the volume edited in 1879. It appears from it that the method 

 of testing by which Cavendish obtained these, results was 

 simply that of physiological sensation; but the figures given 

 in the comparison of iron and sea- water are remarkably exact. 



While the theory of electricity was being established on a sure 

 foundation by the great investigators of the eighteenth century, 

 a no less remarkable development was taking place in the 

 kindred science of magnetism, to which our attention must now 

 be directed. 



The law of attraction between magnets was investigated at 

 an earlier date than the corresponding law for electrically 

 charged bodies. Newton, in the Principia says : " The power of 

 gravity is of a different nature from the power of magnetism. 

 For the magnetic attraction is not as the matter attracted. 

 Some bodies are attracted more by the magnet, others less ; most 

 bodies not at all. The power of magnetism, in one and the same 



* G. B. Beccaria, DdV ehttridsmo artificiale e natural*, Turin. 1753, p. 113. 

 + Phil. Trans. Ixvi (1776), p. 196. % Book iii, Prop, vi, cor. 5. 



