ELECTRIC LIGHT. 103 



siderable distance, and to which it gives the sensation of a 

 shock, rather than that of a spark. When the shock is 

 passed along its surface, the fluid leaves an indelible mark, 

 similar to its effect upon glass. 



Micaceous Schistus gave a spark, which was ramified upon 

 the surface of the stone, and somewhat more coloured than 

 in the Chlorite. It was scarcely phosphorescent, except in 

 the track of the electric fluid. 



Argillaceous Genus. 



Alum affords a purple spark, which is rather ramified upon Argillaceous 

 its surface. It is luminous through its whole substance, Z^^^^' 

 when the explosion is made above its surface ; but it is shat- 

 tered to pieces when the shock is passed through it. 



Pipe-clay gave a spark, and was luminous by the explo- 

 sion ; but when it was formed into tobacco-pipes, and 

 baked, it was scarcely if at all luminous, though it con- 

 tinued to afford sparks. 



A greenish, foliated clay, found near the surface of the 

 ground at Derby, gave a tolerably good spark, and was very 

 phosphoric by the explosion taken above it. The luminous 

 track left by the electric fluid, when the points of the dis»i 

 chargers rested upon the surface, continued several minutes. 



A blueish clay*, dug at Wisbech, and provincially termed 

 Gault, or Golt, affords a spark, and becomes luminous by 

 the explosion. 



Slate clay, or Shale, affords a spark, and is luminous ; but 

 from trials made with different specimens, it appears to lose 

 its power of absorbing light in proportion as it becomes 

 bituminous t. 



Slates. — All the slates afforded sparks, and absorbed 



* This clay, which is frequently dug in the Fens, near Wisbech, con- 

 tains sulphur, and if, when fresh dug, it be held before a fire, it gives 

 out a strong suffocating odour of sulphurous acid. 



t At the time of writing the above observation on bituminous shale 

 I had not the most distant recollection of a paper on the light of 

 natural phosphor!, by M. Carradori, translated in an early number of 

 the Philosophical Journal, where it is mentioned, that luminous rotten 

 wood, and other such like substances, "become phosphorescent in 

 proportion as they have lost their inflammable piinciple, and that the 

 property of absorbing, and retaining the light, depends on that cir- 

 cumstaace."— Nicholson's Journal, 410 scries, Vol. II p. 135. 



electrie 



