Other substances besides English, German, and uranium glass, and Becquerel's luminous sulphides, are also phosphorescent. The rare mineral Phenakite (aluminate of glucinum) phosphoresces blue; the mineral Spodumene (a silicate of aluminium and lithium) phosphoresces a rich golden yellow; the emerald gives out a crimson light. But, without 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 molecular discharge will be directed on it from below upward. On darkening 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 coloring-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 prepared 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. lvii., p. 50, 1859.