36 PHOSPHORESCENCE 
nutes, and increases in brilliancy as the tempera- 
ture rises. 
Potassium also becomes incandescent when em- 
ployed in the preparation of boron, as must have 
been remarked by any chemist who has prepared 
this metalloid. For this purpose vitrified boracic 
acid, in fine powder, and potassium are heated in 
a metallic tube. 
In fact, light, often accompanied by heat, is 
evolved, wherever chemical action is very intense. 
For instance, when sulphur and lead are melted 
together, light is produced whilst the combination 
of these two substances takes place. The same 
is remarked when phosphorus and iodine act upon 
each other: the experiment is very striking, and 
occurs when small quantities of phosphorus are 
covered over with iodine, at the ordinary tempe- 
rature of the atmosphere. In a short time the 
whole takes fire spontaneously. I have seen the 
same occur when a crystal of nitrate of copper 
was enveloped in a thin sheet of tin. 
When arsenic or antimony are thrown into chlo- 
rine gas at the ordinary temperature, the metals 
(which must be in fine powder) burst into flame 
while combining with the chlorme. When caustic 
baryta is placed in a capsule and concentrated 
sulphuric acid poured upon it, the baryta becomes 
incandescent. 
If a drop of water fall into a bottle of anhydrous 
