EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 163 



planes of cleavage, and a nearly complete comprehension 

 of the internal structure of crystalline substances was soon 

 the result. Here we see how much more was due to the 

 reasoning powers of the philosopher, than to an accident 

 which must often have happened to other persons. 



In a similar manner, a purely fortuitous occurrence led 

 Malus to discover the polarization of light by reflection. 

 The phenomena of double refraction had, of course, been 

 long known, and when engaged in Paris in 1808, in 

 investigating the character of light thus polarized, Malus 

 chanced to look through a double refracting prism at the 

 light of the setting sun, reflected from the windows of the 

 Luxembourg Palace. In turning the prism round, he w T as 

 surprised to find that the ordinary image disappeared at 

 two opposite positions of the prism. He remarked that the 

 reflected light behaved exactly like light which had been 

 already polarized by passing through another prism. He 

 was induced to test the character of light reflected under 

 other circumstances, and it was eventually proved that 

 polarization is connected by invariable laws with the act of 

 reflection. Some of the most general laws of optics, pre- 

 viously 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, or friction al electricity, had presented them- 

 selves as exceptional and unaccountable deviations from 

 the ordinary course of Nature. Accident must, of course, 

 have first directed attention to such phenomena, but how 

 few of those who witnessed them had any conception of 

 the all-pervading power thus manifested. The very 

 existence of the so-called 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 is also said 

 to have been accidentally discovered by Nicholson in 1801, 



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