286 PHOSPHORESCENCE. SECT. XXVm. 



of ships. In 1831 the French officers at Algiers were 

 surprised to see brushes of light on the heads of their 

 comrades, and at the points of their fingers, when they 

 held up their hands. This phenomenon was well known 

 to the ancients, who reckoned it a lucky omen. 



Many substances in decaying emit light, which is at- 

 tributed to electricity, such as fish and rotten wood. 

 Oyster shells, and a variety of minerals, become phos- 

 phorescent at certain temperatures, when exposed to 

 electric shocks or friction : indeed most of the causes 

 which disturb molecular equilibrium give rise to phos- 

 phoric phenomena. The minerals possessing this prop- 

 erty are generally colored or imperfectly transparent ; 

 and though the color of this light varies in different sub- 

 stances, it has no fixed relation to the color of the min- 

 eral. An intense heat entirely destroys this property, 

 and the phosphorescent light developed by heat has no 

 connection with light produced by friction, for Sir David 

 Brewster observed that bodies deprived of the faculty of 

 emitting the one are still capable of giving out the other. 

 Among the bodies which generally become phosphores- 

 cent when exposed to heat, there are some specimens 

 which do not possess this property, wherefore phospho- 

 rescence cannot be regarded as an essential character of 

 the minerals possessing it. Sulphuret of calcium, known 

 as Canton's phosphorus, and the sulphuret of barium, or 

 Bologna stone, possess the phosphorescent property in 

 an eminent degree, and M. Edmond Becquerel has shown 

 that on these substances a very remarkable phosphores- 

 cent effect is produced by the action of the different 

 rays of the solar spectrum. In former times Beccaria 

 stated that the violet ray was the most energetic, and 

 the red ray the least so, in exciting phosphoric light. M. 

 Becquerel has shown that two luminous bands separated 

 by a dark one are excited by the solar spectrum on pa- 

 per covered with a solution of gum-arabic and strewed 

 with powdered sulphuret of calcium. One of the lu- 

 minous bands occupies the space under the least refran- 

 gible violet rays, and the other that beyond the lavender 

 rays, so that the dark band lies on the part under the 

 extreme violet and lavender rays. When the action of 

 the spectral light is continued, the whole surface beyond 



