14 _ PHOSPHORESCENCE 
phur, to an intense heat for one hour.* It can 
also be formed by heating gypsum with charcoal. 
Some diamonds, but not all, possess the same 
property, and manyother substances have been ob- 
served to be more or less phosphorescent in the 
same circumstances, that is, after msolation.+ 
Landrin (Dict. de Min.) asserts that radiated 
sulphate of baryta, certain natural fluorides, rock- 
salt, amber (succinum), and quartz, become lumi- 
nous for a few instants after they have been ex- 
posed to the sun. 
Walls that have been painted or whitewashed 
with lime, are apt to become luminous at night 
after they have received the action of the sun’s 
rays in the daytime. Whitewashed houses are, 
on account of their phosphorescent quality, visible 
at a great distance on the darkest nights. 
It was natural enough that the action which the 
coloured rays of the solar spectrum exercise upon 
these substances, that become phosphorescent after 
insolation, should be early investigated; and in 
1775, Wilson published his ‘ Series of Experiments 
on the Phosphori,’ in which paper he asserts that 
the most refrangible rays of the solar spectrum 
determine a vivid phosphorescence in sulphuret 
of calcium (‘ Canton’s phosphorus”), whilst those 
rays which are the least refrangible—i.e. those 
situated near the red heht of the spectrum—cause 
* Philosophical Transactions, 1768. + See Chapter VI. 
