28 PHOSPHORESCENCE 
friction of mercury in the vacuum of the baro- 
meter tube produces a phosphorescent light. 
When oxygen gas or air are compressed sud- 
denly in a piston, heat alone is produced. ‘The 
light seen in this experiment is owing to the com- 
bustion of some of the oil of the piston, as proved 
by Thenard. When the piston is imbibed with 
water, no light is perceived; and M. Saissy, of 
Lyons, has proved that oxygen alone, by its com- 
burent power, is the cause of this hght. 
If chlorate of potash, fluor-spar, feldspar, sugar, 
etc. be struck in the dark, or ground down in a 
mortar, they present very vivid phosphoric radia- 
tions. With crystallized substances which are 
cleavable, i.e. easily divided into thin laminze, this 
phosphorescence is very remarkable ; with sugar, 
for instance, each fissure produced by the shock of 
the pestle gives birth to a streak of light which 
lasts for an instant, and when a certain quantity 
of any of these substances is ground down rapidly 
in a mortar, the whole mass appears as if on fire. 
This beautiful phenomenon is exceedingly striking 
when transparent feldspar is experimented upon. 
It has lately been discovered that dry hypophos- 
phites of lime, soda, etc., become phosphorescent 
when shaken or stirred in the dark. (Note.— 
These salts are apt to explode violently when 
evaporated to dryness at too high a temperature. 
Tuson, in ‘Chemical News,’ August, 1860.) 
