382 SCIENTIFIC AMUSEMENTS. 



fluorescent qualities), or of the allied dyestuff eosin, in the track of 

 the invisible ultra-violet rays thus separated from the visible rays, 

 the particular coloured light emitted by each substance respectively 

 is seen to advantage, a soft glow of light of peculiarly beautiful 

 appearance becoming visible, somewhat resembling the luminosity 

 of a mass of yellow phosphorus exposed to air in a dark room, but 

 varying in tint. 



The term " fluorescence " is derived from the circumstance that 

 " fluor spar " (so named from its employment as a flux in certain 

 metallurgical processes vide also Expt. 271) was one of the first 

 bodies in which this property was noticed ; it is curious that this 

 same substance is also capable of setting up, under suitable con- 

 ditions, an apparently opposite phenomenon, sometimes spoken of 

 as phosphorescence by heat. 



Expt. 403. To render Fluor Spar Luminous by heating. 

 Heat a stout iron fireshovel over a good fire until it is nearly hot 

 enough to glow dull red in the dark, but not quite ; strew over it 

 some coarsely powdered fluor spar, and then examine it in a per- 

 fectly dark room; the particles of spar will glow with a faint lambent 

 light, somewhat resembling that emitted by phosphorus. 



In this case the fluor spar appears to absorb the radiant heat 

 rays of refrangibility too low to be visible and emit them again as 

 rays of higher refrangibility so as to be visible, the action being 

 somewhat akin to that taking place in Expt. 377, where invisible 

 radiant heat is concentrated by a burning glass so as to heat a 

 solid body placed at the focus sufficiently to render it red hot, i.e., 

 to cause it to emit visible light of higher refrangibility than the 

 rays passing through the lens. In somewhat similar fashion 

 certain fluorescent bodies can be made to fluoresce visibly when 

 exposed to light of a less degree of refrangibility than that emitted 

 whilst fluorescing. Thus if a beam of deep red light be isolated 

 from arc light, &c., by means of a prism and screens, and allowed 

 to fall on naphthalene red (a coal tar dyestuff) this latter fluoresces 

 an orange yellow light; chlorophyll (the green colouring matter 

 of vegetation) behaves similarly, the light emitted whilst fluores- 

 cing being less markedly orange than in the case of naphtha- 

 lene red, but more so than the incident deep red light employed. 

 The mineral known as chlorophane (a variety of fluor spar) 

 is capable of fluorescing emerald green when illuminated by 

 dark heat radiation of too low refrangibility to be visible at all. 



Expt. 404. Phosphorescence. The term phosphorescence is 

 applied to two different kinds of phenomena which are not at all 

 akin, excepting in the general result that a more or less feeble 

 light is emitted by the phosphorescent body. When a piece of 



