224 DIAMONDS. 



when it passes out into the air, giving rise to the lightnings and corus- 

 cations for which the diamond is supreme above all other gems. 

 The following table gives the refractive indices of diamonds and 



other bodies : 



Refractive indices for the D line. 



Chromate of lead 2. 50-2. 97 



Diamond 2.47-2.75 



Phosphorus 2. 22 



Sulphur 2.12 



Euby - 1.78 



Thallium glass - - - 1. 75 



Iceland spar 1. 65 



Topaz 1.61 



Beryl 1.60 



Emerald 1. 59 



Flint glass ' 1. 58 



Quartz 1. 55 



Canada balsam 1. 53 



Crown glass 1. 53 



Fluor-spar 1. 44 



Ice 1.31 



According to Dr. Gladstone, the specific refractive energy, '" ~ , will 



(aj 



be for the D line 0.404, and the refraction equivalent, P ^'~ .., will 



be 4.82. 



After exposure for some time to the sun many diamonds glow in a dark 

 room. Some diamonds are fluorescent, appearing milky in sunlight. 

 In a vacuum, exposed to ahigh tension current of electricity, diamonds 

 phosphoresce of different colors, most South African diamonds shining 

 with a bluish light. Diamonds from other localities emit bright blue, 

 apricot, pale blue, red, yellowish green, orange, and pale green light. 

 The most phosphorescent diamonds are those which are fluorescent in 

 the sun. One beautiful green diamond in my collection, when phos- 

 phorescing in a good vacuum, gives almost as much light as a candle, 

 and you can easily read by its rays. The light is pale green, tending 

 to white. 



CONVERSION OF DIAMOND INTO GRAPHITE. 



I will now draw your attention to a strange property of the diamond, 

 which at first sight might seem to argue against the great permanence 

 and unalterability of this stone* It has been ascertained that the cause 

 of phosphorescence is in some way connected with the hammering of 

 the gaseous molecules, violently driven from the negative pole, on to 

 the surface of the body under examination; and so great is the energy 

 of the bombardment that impinging on a piece of platinum, or even 

 iridium, the metal will actually melt. When the diamond is thus bom- 

 barded in a radiant matter tube the result is startling. It not only 

 phosphoresces, but assumes a brown color, and when the action is long 

 continued becomes almost black. 



