Rainbotf 1 Phosphorescence. 139 



guished sooner in yellow or orange rays than if placed nearly in 

 a dark chamber. If these experiments are repeated, with a 

 prismatic spectrum the same results are always obtained." 



The Newtonian theory that light consisted in an emission 

 of minute particles shot forth with great velocity, led the 

 older philosophers to conceive that these phosphorescent 

 bodies had bottled up the light, something like the cucum- 

 ber of Laputa, and gave it back again, as the celebrated sage 

 *)f that island desired to make the cucumbers do. Such 

 notions, however, and the somewhat similar one, that they 

 absorb light, as sponges absorb water, are quite inconsistent 

 with some important facts, and with the undulatory theory of 

 light, which may now be considered as well established. M. 

 Becquerel observes that such comparisons are incorrect, inas- 

 much as the light emitted usually differs from that which 

 excites the phosphorescent body. Phosphorescence is generally 

 an emission of light-waves of less velocity than those of the 

 rays which excited it. 



The most luminous of these phosphori are the sulphides of 

 alkaline earths, calcium, barium, and strontium. The first 

 gives Canton's phosphorus emitting a yellow or a green light ; 

 the second, the Bologna phosphorus, for the most part orange. 

 M. Becquerel remarks that " these substances, when well pre- 

 pared, will shine in darkness for many hours after their ex- 

 posure to solar action, decreasing, however, rapidly in lustre 

 in the first moments, and then growing weaker more slowly. 

 Their light is emitted in vacuum as well as in gases, and their 

 action is not accompanied by any chemical effect ; it is the 

 result of a temporary physical modification. Amongst the 

 metallic sulphides, those of strontium and barium exhibit the 

 greatest vivacity of luminous emission, and those of calcium 

 yield the greatest variety of colours." 



A sulphide of zinc, formed in a particular way, is as phos- 

 phorescent as the sulphides of the alkaline earths. Other 

 metallic sulphurets do not exhibit the property, not even those 

 of the alkaline metals, and the other compounds of barium, 

 strontium, and calcium, excepting their selenides, do not 

 manifest energetic action of this nature. Following the sul- 

 phides just mentioned, come minerals, such as certain diamonds, 

 especially those of a yellow tint, and most specimens of fluoride 

 of calcium. The variety of calcic fluoride called chlorophane 

 becomes very luminous by insolation, emitting a slightly bluish 

 green tint of light. The reason why particular diamonds, or 

 chlorophanes, are phosphorescent, while others do not exhibit 

 that property, is at present unknown. M. Becquerel says, 

 <c the diamond and fluoride of calcium do not exhibit a vivacious 

 lustre, but they remain luminous for a long time. Thus, I have 



