r06 CI-ECTRIC LIGHT. 



Siliceous Lapis Lazuli affords a very good spark, and is luminotis 



jenus. by the shock. 



Egyptian Pebbles, Scotch Pebbles, Felspars, Agates, 

 Calcedonies, Carnelians, and Jaspers, gave hissing purple 

 sparks, and were luminous by the explosion. Several of 

 these substances give out the same odour as Avhen two pieces 

 are rubbed against each other. 



Porphyries and Granites gave a hissing purple spark, and 

 were luminous by the shock, which, when passed upon the 

 surface, produced a very bright track of light, which iu 

 some specimens, especially in a piece of whitish Granite, 

 continued luminous for several minutes. 



Pudding Stones gave a similar hissing spark ; and the oval 

 pebbles being more luminous than the siliceous sand, iu 

 which they are imbedded, were readily distinguished in the 

 dark when the shock was passed above the surface of the 

 stone. 



Mochoas gave very good sparks from the arborescent 

 parts, but only a hissing stream from the stone itself, which 

 is slightly luminous by the shock, but affords a bright track 

 of light betAveen the ends of the discharging rods. 



Tlie Yorkshire Stone, which is used for flat pavement, 

 gives a purple spark, and seems to become luminous by the 

 electric explosion, in proportion as it partakes of a calca- 

 reous nature, for those specimens which are verging toward 

 Biicaceous schist, (and in which I have found lamina of sul- 

 phur nearly the tenth of an inch thick,) arc scarcely at all 

 luminous. 



l^umice Stone on some parts of its surface gave only a 

 hissing stream, but en others very good sparks, which ap- 

 peared to penetrate through its substance, as if it contained 

 some metallic particles within it. The shock rendered it 

 slightly luminous, but it afforded a very bright track of 

 light along its surface, between the ends of the dischargers. 

 The semivitrified ashes of a Haystack, which was con- 

 sumed by spontaneous combustion, gave only a hissing 

 stream, and was slightly luminous by the electric explosion ; 

 but when the shock was passed upon its surface it afforded a 

 bright track of light. 



Various kinds of Glass are not luminous, neither do they 



give 



