236 THE PRINCIPLES OF SCIENCE. 



heavy glass, but solids and liquids, acids and alkalis, 

 oils, water, alcohol, ether, all possess this power ; but he 

 was not able to detect its existence in any gaseous sub- 

 stance. His thoughts cannot be restrained from running 

 into curious speculations as to the possible results of the 

 power in certain cases. ' What effect,' he says, ' does this 

 force have in the earth where the magnetic curves of the 

 earth traverse its 'substance 1 Also what effect in a mag- 

 net "? ' And then he falls upon the wholly original notion 

 that perhaps this force tends to make iron and oxide of 

 iron transparent, a phenomenon never previously or since 

 observed. We can meet with nothing more instructive 

 as to the course of mind by which great discoveries are 

 made, than these records of Faraday's patient labours, 

 and his varied success and failure. Nor are his unsuccess- 

 ful labours upon the relation of gravity and electricity 

 less interesting, and worthy of study. 



Throughout a large part of his life, Faraday was pos- 

 sessed by the idea that gravity cannot be unconnected 

 with the other forces of nature. On March ipth, 1849, 

 he wrote in his laboratory book ' Gravity. Surely this 

 force must be capable of an experimental relation to elec- 

 tricity, magnetism, and the other forces, so as to bind it 

 up with them in reciprocal action and equivalent effect 11 .' 

 He filled twenty paragraphs or more with reflections and 

 suggestions, as to the mode of approaching the subject 

 by experiment. He anticipated that the approach of one 

 body to another would develope electricity in them, or 

 that a body falling through a conducting helix would 

 excite a current changing in direction as the motion was 

 reversed. * All this is a dream,' he remarks; 'still ex- 

 amine it by a few experiments. Nothing is too wonderful 

 to be true, if it be consistent with the laws of nature ; 



h See also his more formal statement in the ' Experimental Researches 

 in Electricity,' 24th Series, 2702, vol. iii. p. 161. 



