One of the investigations with which Cavendish occupied himself was a comparison between the conducting powers of different materials for electrostatic discharges. The question hall been first raised by Beccaria, who had shown[1] 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 Royal Society in 1775,[2] 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,[3] 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
