160 ELECTRICAL EXCITEMENT. 



light, that the minerals should be first warmed near a 

 a fire. From this it may be infered that the luminous 

 appearance is of a similar character to that of fiuor 

 spar, and of numerous other earthy minerals, which, 

 when exposed to heat, phosphoresce with great brilliancy. 

 Phosphorescent glow can also be excited in similar 

 bodies by electricity, as was first pointed out by Father 

 Beccaria, and confirmed by Mr. Pearsall.* These 

 effects, it must be remembered, are distinct from the 

 electric spark manifested upon breaking white sugar in 

 the dark, or scratching sulphuret of zinc. 



In the instances adduced there is not necessarily any 

 exposure to the sunshine required. It is probable that 

 two, if not three, distinct phenomena are concerned in 

 the cases above quoted, and that all of them are distinct 

 from animal phosphorescence, or the luminous appear- 

 ance of vegetables. They, however, certainly prove, 

 either that light is capable of becoming latent, or that it 

 is only a condition of matter, in which it may be made 

 manifest by any disturbance of the molecular forces. 



* Phosphorescence of the Diamond : by M. Keiss (Revue Scien- 

 tifique et Industrielle, vol. xxiii. p. 185). " The diamond, phos- 

 phorescent by insulation, lost rapidly its phosphorescence when 

 submitted to the action of the red rays of the solar spectrum. 

 On the contrary, the blue rays are those which render the diamond 

 the most luminous in the dark. It is probable that the phospho- 

 rescence produced by heat is equally diminished by the action of 

 the red rays of the solar spectrum." Giovanni Battista Beccaria 

 published his experiments in 1769. See Priestley's History of 

 Electricity ; and On the Effects of Electricity upon Minerals which 

 are Phosphorescent by Heat ; and further Experiments on the com- 

 munication of Phosphorescence and Colour to bodies of Electricity ; 

 by Thomas J. Pearsall. Journal of the Royal Institution of 

 Great Britain, Oct. 1830, Feb. 1831. These two memoirs contain 

 the most complete set of experiments on this subject which have 

 yet been made ; see Placidus Heinrich, Phosphorescenz der Korper, 

 vol. iv. ; Gmelin's Handbuch der Chemie, part 1.; On the Phos 

 phorcscence of Minerals, Brewster : Edinburgh Philosophical 

 Journal, vol. i. p. 137. ; The Aerial Noctiluca, or some New 

 Phenomena, and a process of a factitious self -shining substance .- 

 B gyle's Works, vol. iv. 



