30 PHOSPHORESCENCE 



nates the glass globe. M. Pontus has repeated the 

 experiment often, and says that the production of 

 this spark is a sure sign that congelation is about 

 to happen. 



It is well known to chemists that arsenious acid 

 exists under two distinct molecular modifications 

 discovered by M. Guibourt, viz. the transparent 

 acid and the opaque acid. Professor H. Kose, of 

 Berlin, has shown that when the transparent va- 

 riety is dissolved in a hot solution of diluted 

 hydrochloric acid, and the dissolution allowed to 

 cool, the opaque variety is deposited in crystals, 

 and each crystal, as it forms, is accompanied by 

 an emission of light.* 



The same emission of light is observed when 

 certain oxides, whilst heated in a crucible to a 

 given temperature, undergo a peculiar molecular 

 change which occasions a modification of their 

 chemical properties. A phosphoric radiation, a 

 sort of incandescence, is remarked the instant 

 that this change takes place. The substances 

 that are remarkably phosphorescent during this 

 molecular change occasioned by heat are, alu- 

 mina, chromic oxide, oxide of zirconium, tantalic 

 acid, titanic acid, the acids of the new metal nio- 

 bium, peroxide of iron, and some others. In the 

 mineral gadolinite, this phenomenon is very well 



* See Rose's paper on this in the c Annalen der Physik und 

 Chemie,' 1835. 



