BY HEAT. 21 



Almost any substance,, whether organic or mi- 

 neral, if a non-conductor of electricity, becomes 

 more or less phosphorescent when it is thrown 

 upon a molten bath of the easily-melting alloy of 

 D'Arcet. Indeed, Wedgwood published an ela- 

 borate paper upon this subject in the ' Philoso- 

 phical Transactions' for 1792. He experimented 

 upon an extensive variety of substances both 

 mineral and organic, by reducing the body to a 

 moderately fine powder, and sprinkling it by 

 small portions at a time on a thick plate of iron 

 heated just below visible redness, and removing 

 the whole to a perfectly dark place. He has given 

 a prodigious list of substances which appear lu- 

 minous for a few instants when submitted to this 

 treatment. 



When a body which is known to be phospho- 

 rescent by heat loses, from some undetermined 

 cause, its phosphoric property, the latter can be re- 

 stored to it by means of electricity, as we have 

 seen in the foregoing chapter regarding substances 

 which are phosphorescent after insolation. For ex- 

 ample, certain diamonds which cannot be made to 

 give out any phosphoric radiation by heat, will do 

 so after one or two electric discharges have been 

 passed over them. This curious fact was made 

 known by the German savant Grothuss. 



Pearsall (in ' Journal of Royal Institution/ 

 vol. i.) has described experiments proving that a 



