530 THE PRINCIPLES OF SCIENCE. [chap. 



to observe the planes of cleavage, and the discovery of the 

 internal structure of crystalline substances was the result. 

 Here we see how much more was due to the reasoning 

 power of the philosopher, than to an accident which must 

 often have happened to other persons. 



In a similar manner, a fortuitous occurrence led Mains 

 to discover the polarisation of light by reflection. The 

 phenomena of double refraction had been long known, and 

 when engaged in Paris in 1 808, in investigating the cha- 

 racter of light thus polarised, Mains chanced to look 

 through a double refracting prism at the light of the setting 

 sun, reflected from the windows of the Luxembourg Palace. 

 In tuniing the prism round, he was surprised to find that 

 the ordinary image disappeared at two opposite positions 

 of the prism. He remarked that the reflected liglit behaved 

 like light which had been polarised by passing through 

 another prism. He was induced to test the character of 

 light reflected under other circumstances, and it was 

 eventually proved that polarisation is invariably -connected 

 with reflection. Some of the general laws of optics, 

 previously unsuspected, were thus discovered by pure 

 accident. In the history of electricity, accident has had a 

 large part. For centuries some of the more common 

 effects of magnetism and of frictional electricity had pre- 

 sented tliemselves as unaccountable deviations from the 

 ordinary course of Nature. Accident must have first 

 directed attention to such phenomena, but how few of 

 those who witnessed them had any conception of the all- 

 pervading character of the power manifested. The very 

 existence of galvanism, or electricity of low tension, was 

 unsuspected until Galvani accidentally touched the leg of 

 a frog with pieces of metal. The decomposition of water 

 by voltaic electricity also was accidentally discovered by 

 Nicholson in 1801, and Davy speaks of this discovery as 

 the foundation of all that had since been done in electro- 

 chemical science. 



It is otherM'ise with the discovery of electro-magnetism. 

 Oersted, in conmion with many others, had suspected the 

 existence of some relation between the magnet and 

 electricity, and he appears to have tried to detect its exact 

 nature. Once, as we are told by Hansteen, he had em- 

 ployed a strong galvanic battery during a lecture, and at 



