1 4 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 many other substances have been ob- 

 served to be more or less phosphorescent in the 

 same circumstances, that is, after insolation. f 



Landrin (Diet, 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 light of the spectrum cause 



* Philosophical Transactions, 1768. f See Chapter VI. 



