BY HEAT. 21 
Almost any substance, whether organic or mi- 
neral, if a non-conductor of electricity, becomes 
more or less phosphorescent when it 1s 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 undetermmed 
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 
