Electroluminescence 291 



and two books, Dell'Elettricismo Artificiale e Naturale (Turin, 

 1853) and Lettere dell'Elettricismo (Bologna, 1758) , An English 

 edition appeared in 1776, A Treatise upon Artificial Electricity. 

 One of Beccaria's goals was to discover the relation between com- 

 mon fire and electric fire; another was to find out what part elec- 

 tricity played in various luminescences. He made the statement 

 that " common fire propagates itself with most difficulty, through 

 such bodies as refuse to conduct the electric fire, and most easily 

 through those which can conduct it." Beccaria proposed to prove 

 this by such arguments as " The common as well as the electric fire, 

 are speedily dissipated in dilated air," etc. 



He first studied the barometer light, noting that a little air must 

 be present and, like Hauksbee, that a ring of light appears at the 

 mercury surface when the mercury moves downward. An electric 

 discharge through the Cavendish double barometer gave splendid 

 effects and bubbles of air rising in the mercury looked like the 

 lighting globes seen during an auroral display. The color of the 

 light depended on the " dilation " of the air and was often reddish, 

 as in an aurora borealis. 



He noted the flashes of light in evacuated glass globes when 

 rubbed, but was particularly interested in the cause of the light in 

 evacuated glass vessels when broken. Priestley (1769: 191) has de- 

 scribed the latter experiments as follows: 



Signor Beccaria observed that hollow glass vessels, of a certain thin- 

 ness, exhausted of air, gave a light when they were broken in the dark.^^ 

 By a beautiful train of experiments, he found, at length, that the lumi- 

 nous appearance was not occasioned by the breaking of the glass, but 

 by the dashing of the external air against the inside, when it was broke. 

 He covered one of these exhausted vessels with a receiver, and letting 

 the air suddenly on the outside of it, observed the very same light. This 

 he calls his new invented phosphorus. {Lettere dell'Elettricisino, 354 

 etc., 1758.) 



The light comes from electrification of the glass as the air rushes 

 over the surface and is a true electroluminesce. Beccaria found that 

 solid glass spheres like " Batavic drops " " or " Bologna bottles " do 

 not luminesce when broken, either in air or in vacuum. 



"J. Burke (1895), at the suggestion of J. J. Thomson, repeated Beccaria's experi- 

 ments and confirmed his results but could not explain all the phenomena observed, 

 especially the fact that luminescence appeared to be associated with the broken frag- 

 ments of glass. 



" Presumably " Prince Rupert drops," allegedly discovered by Prince Robert Rupert 

 of Bavaria, but probably known since glass blowing began. They were mentioned by 

 Samuel Pepys in his Diary, January 13, 1662. as " chymicall glasses, which break all 

 to dust by breaking off a small end; which is a great mystery to me." Knolglaser were 



