ON LIGHT. 393 



scarcely possible such singularities should stand in 

 no natural connexion. Eetvveen two of the cases ad- 

 duced the connexion had been proved by himself. It 

 remained to enquire whether the third could be brought 

 into obvious relation to the other two. Accordingly on 

 the 14th of March 1823, having prepared a long spiral 

 coil of copper wire enclosed in an earthenware tube, 

 furnished with a polarizing reflector at one end and an 

 analyzer at the other; by the kindness of the late Mr 

 Pepys, he was permitted to bring the coil into connexion 

 with the great magnetic combination of the London 

 Institution, consisting of one enormous couj)le, expressly 

 arranged for producing the greatest possible magnetic 

 effect. His expectation was that light would appear in 

 the dark polarized field on making the contact, and be 

 maintained during its continuance. The experiment, 

 however, proved unsuccessful. No direct action upon 

 light could so be made manifest. At a later period, 

 however (1845), ^Y introducing into a similar coil a 

 certain highly refractive glass consisting chiefly or wholly 

 of borate of lead, as well as a variety of other solids 

 and liquids (water among others). Professor Faraday 

 succeeded in communicating, temporarily, and during 

 the continuance of the passage of the current, the pro- 

 perty in question to them. 



(169.) Biaxal Crystals. By far the greater number 

 of crystallized substances do not present that single sym- 

 metry (symmetry on all sides of a single central line or 

 axis), which we have spoken of as indicative of a single 

 axis of double refraction, and of a spherical propagation 



