236 THE INTELLECTUAL RISE IN ELECTRICITY. 



every other particle with a force whose direction is that 

 of a line joining the two and whose magnitude ... is 

 inversely as the square of their distance from each other." 

 Light radiation equally moves in a straight line, and its 

 intensity varies as the square of the distance between the 

 surface on which it falls and the source of radiation. 

 Those are the laws of the phenomena stated in the fore- 

 going quotation. Put the two together, and the result is 

 Michell's sixth law, first expressed in 1750 : "The Attrac- 

 tion and Repulsion of Magnets decrease as the squares of 

 the distances from the respective Poles increase." 1 

 Neither Sarpi, who had so narrowly studied the human 

 eye and had dived into the deeps of all existing optical 

 knowledge, nor even Porta who had followed -him, could 

 perhaps have expressed that law as Michell did a couple of 

 centuries later; but they knew how the light of a candle 

 increased and diminished, and they saw the resemblance 

 between this and the increase and diminution of the virtue 

 around the poles of a magnet. How better could they 

 state their knowledge than by making the comparison and 

 pointing out the resemblance? 



But let us go a little further. Whenever Porta becomes 

 grave and states an important fact, he shortly afterwards 

 relaxes and tells about u jokes of the magnet with which 

 we often exhilarate friends," and such parts of the book 

 are undoubtedly his own. He is particularly fond of the 

 iron-filings experiment, and, after stating the similarity of 

 the sphere of magnetic attraction to the candle radiations, 

 he recurs to it. This time he places on a table two masses 

 of magnet fragments, and, holding a lodestone in each 

 hand under the table, he makes the particles move, as he 

 fancied, to represent contending armies. The individual 

 grains rise up like erected spears, and advance and re- 

 treat and enter into "deadly struggles, now conquering, 

 now conquered, now with arms raised, now lowered;" and 

 then, in the midst of the play, he remarks, u the nearer 



'Michell, J.: A Treatise of Artificial Magnets, London, 1750. 19. 



