ELECTRIC LIGHT. 



cence, and its slightly effervescing with acids^ I suspect io 

 contain more calcareous earth in its composition than the 

 others. 



Queen's ware, glazed, gives a good spark, which is flame 

 coloured, and radiated on its surface; but it is not phos- 

 phoric. When fractured, the unglazed surface affords a 

 purple spark, and is luminous by the shock. 



AH the different kinds of Pottery-ware which I tried gave 

 the same results, except slight varieties in the colour of the 

 sparks: and a common dirty white ware, which was lumi- 

 nous on its glazed surface when the shock was passed above 

 it. From what I have hitherto observed, I am induced to 

 believe that all glazing, in which a metallic oxide is used, is 

 not phosphoric, but gives a good spark. 



All unglazed Pottery is luminous by the explosion, and 

 gives a vivid track of light when the dischargers rest upon its 

 surface. 



Siliceous Genus. 

 Rock Crystals were all phosphoric by the explosion ; and SUiceou* 

 some of them that had two or three particles of ore upon Z^^^^' 

 their surfaces, were transparent by the spark when it passed 

 from the ore to the knob of the discharging rod, otherwise 

 the crystals gave no sparks, but merely a hissing stream. A 

 rhombic crystal was rendered luminous through its whole 

 substance by the explosion, retaining its light several minutes. 

 At the instant after the explosion it emitted a red light, but 

 which very soon became white. 



Siliceous sand, washed and dried, was not luminous, ex- 

 cept where the points of the dischargers were in contact 

 with it. 



Quartz is phosphoric, and shines with a uniform dull 

 white light. It gives a purple stream instead of a spark. 

 After the explosion it affords the same odour as when two 

 pieces are rubbed together. 



Flints atlbrd small purpl« sparks, both from the external 

 coat and the surface of the fracture. The explosion does 

 not render them so luminous as Quartz, but the external 

 coat is more phosphorescent than the fracture, especially in 

 the track of the discharge. 



Vol. XVI.-^Fkb. 1807. K Lapis 



105 



