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. Rose, of 
Berlin, has shown that when the transparent va- 
riety 1s 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, 1s 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, 1s 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 ‘ Annalen der Physik und 
Chemie,’ 1835. 
