ELECTRICITY AND LIGHT. 57 



The great Epicurus speculated on the " plastic nature" 

 of atoms,, and attributed to this nature the power they 

 possess of arranging themselves into symmetric forms. 

 Modern philosophers satisfy themselves with attraction, 

 and, reasoning from analogy, imagine that each atom 

 has a polar system. 



Electricity, and light, and heat, exert remarkable 

 powers, and accelerate or retard crystallisation according 

 to the conditions under which these forces are brought 

 to bear on the crystallising mass. We have recently 

 obtained evidence which appears to prove that some form 

 of magnetism has an active influence in determining the 

 natural forms of crystals, and we discover that magnetism 

 exerts a peculiar influence in relation to the optic axes of 

 crystals, which is not exerted in lines at right angles to 

 these. Electricity appears to quicken the process of 

 crystalline aggregation to collect more readily together 

 those atoms which seek to combine to bring them all 

 within the limits of that influence by which their sym- 

 metrical forms are determined ; and strong evidence is 

 now afforded, in support of the theory of magnetic 

 polarity, by the refined investigations of Faraday and 

 Pllicker, which prove that magnetism has a directing 

 influence upon crystalline bodies.* 



It has been found that crystals of sulphate of iron, 

 slowly forming from a solution which has been placed 

 within the range of sufficiently powerful magnetic force, 

 dispose themselves along certain magnetic curves, such 

 as are formed around a magnet by steel filings ; whereas 

 the crystals of the Arbor Dianse, or silver tree, forming 



* On the Magnetic Relations of the Positive and Negative Optic 

 Axes of Crystals, by Professor Pliicker, of Bonn. Philosophical 

 Magazine, No. 231 (3rd Series), p. 450. Experimental Researches 

 on Electricity ; On the Crystalline Polarity of Bismuth and other 

 bodies, and on its Relation to the Magnetic form of Force : by 

 Michael Faraday, Esq., F.K.S. Transactions of the Royal Society 

 for 1848 



