produced by the action of Liylit upon Bodies. 525 



sulphurets of the alkaline earths, sucli as the sulphurets of barium, 

 strontium, and calcium; the two first present a greater activity 

 in the emission of light, and the third distinguishes itself by a 

 greater variety in the tints. Afterwards come several kinds of 

 diamonds, most of the varieties of Huoride of calcium, in parti- 

 cular the variety known as chlorophane, and subsequently a 

 great number of minerals and salts ; for the most part, however, 

 these only retain the property of emitting light for a few seconds 

 — rarely a few minutes — after exposure to luminous rays. 

 Amongst the list of these bodies, too, are the compounds of lime ; 

 a great number of the compounds of stx'ontia and barytes ; mag- 

 nesia, soda, and potash, together with many of their compounds ; 

 sal-ammoniac, succinic acid, oxalic acid, borax, &c. Amongst 

 the organic bodies possessing the property in question, are dried 

 paper, silk, sugar, sugar of milk, teeth, &c. Although most of 

 these substances were already known to possess the property of 

 phosphorescence under the action of light, there still remained to 

 be investigated a great many circumstances accessory to the action 

 of luminous radiation, as well as the causes which are introduced 

 in the preparation of artificially phosphorescent bodies in order 

 to bestow upon them the property of presenting rays of different 

 refrangibility. Such are some of the principal points which the 

 author proposed to study, and of the importance of which there 

 can be no difference of opinion ; for, as he himself remarks, " the 

 phsenomeuon of phosphorescence by the action of radiation, is 

 one of the most curious in molecular physics; it tends to elucidate 

 the manner in which luminous vibrations become modified when 

 they strike different substances, thus giving rise, perhaps, to 

 some of the effects upon which the colour of bodies depends." 



After having passed in re^'ievv the principal substances which 

 become phosphorescent by insolation, the author resumes the 

 examination of the sulphurets of calcium and barium, to which 

 the name of artifcial phospliois has been preeminently given. 

 It had been already shown that, according to the mode of 

 preparing these substances, the property of emitting different- 

 coloured lights could be imparted to artificial phosphors; but 

 the causes of these differences had never been specified. M. 

 Becquerel having already been led to suppose that in certain 

 cases the same substance might be endued with different lumi- 

 nous properties according to its physical or molecular condition, 

 has re-examined with great care the different modes of preparing 

 these exceedingly sensitive substances. He experimented prin- 

 cipally upon the different reactions by means of which the sul- 

 phurets of calcium and barium are obtained ; and the result at 

 which he arrived was, that the tint of the phosphorescent light 

 emitted bv these different substances varies not onlv with their 



