i8 THE POPULAR SCIENCE MONTHLY. 



Other substances besides English, German, and iu*anium glass, and 

 Becquerel's luminous sulphides, are also phosphorescent. The rare min- 

 eral Phenakite (aluminate of glucinum) phosphoresces blue ; the min- 

 eral Spodumene (a silicate of aluminium and lithium) phosphoresces a 

 rich golden yellow ; the emerald gives out a crimson light. But, with- 

 out exception, the diamond is the most sensitive substance I have yet 

 met for ready and brilliant phosphorescence. Here is a very curious 

 fluorescent diamond, green by daylight, colorless by candle-light. It 

 is mounted in the center of an exhausted bulb (Fig. 3), and the mo- 

 lecular discharge will be directed on it from below upward. On dark- 

 ening the room you see the diamond shines with as much light as a 

 candle, phosphorescing of a bright green. 



Next to the diamond the ruby is one of the most remarkable stones 

 for phosphorescing. In this tube (Fig. 4.) is a fine collection of ruby- 



pebbles. As soon as the induction-spark is turned on, you will see 

 these rubies shining with a brilliant rich red tone, as if they were 

 glowing hot. It scarcely matters what color the ruby is, to begin 

 with. In this tube of natural rubies there are stones of all colors — the 

 deep-red and also the pale-pink ruby. There are some so pale as to 

 be almost colorless, and some of the highly prized tint of pigeon's 

 blood ; but under the impact of radiant matter they all phosphoresce 

 with about the same color. 



Now the ruby is nothing but crystallized alumina with a little color- 

 ing-matter. In a paper by Ed. Becquerel,* published twenty years 

 ago, he describes the appearance of alumina as glowing with a rich red 

 color in the phosphoroscope. Here is some precipitated alumina pre- 

 pared in the most careful manner. It has been heated to whiteness, 

 and you see it also glows under the molecular discharge with the same 

 rich red color. 



The spectrum of the red light emitted by these varieties of alumina 

 is the same as described by Becquerel twenty years ago. There is one 



* " Annales de Chimie et de Physique," third series, vol. Ivii., p. 50, 1859. 



