330 History of Luminescence 



by to cool: when cold, turn it out of the crucible, and cutting, or break- 

 ing it to pieces, scrape off, upon trial, the brightest parts; which, if good 

 phosphorus, will be a white powder; and may be preserved by keeping it 

 in a dry phial with a ground stopple. 



Canton did not mention the color of the luminescence. He 

 studied the effect of prolonged illumination, of electric sparks,^* 

 which were as good exciters as daylight, of heat (increases light) , 

 of water (slowly destroys) , alcohol (slight effect) and ether (no 

 effect) on the behavior of his phosphor. In another paper he also 

 carried out a few observations on shining fish, which he believed 

 might explain the ' burning of the sea." The heat experiments are 

 of special interest as indicating that moderate rise in temperature 

 increases the light intensity but decreases the duration. Canton 

 came to the above conclusion by comparing two luminescent sam- 

 ples in glass containers at room temperature and at the temperature 

 of boiling water. The material merely parted with the light more 

 quickly at the higher temperature. Canton also observed that two 

 days later, when no light cotild be observed in either sample, a quite 

 bright luminescence appeared in the phosphor not previously heated 

 when placed in boiling water. Six months later the phosphor which 

 had previously lost its light in boiling water would luminesce again 

 if placed on a hot iron plate. The experiment was confirmed by 

 Nathaniel Hulme (1732-1807) in 1801. This phenomenon is none 

 other than thermoluminescence (see Chapter IX) and again illus- 

 trates the close connection between phosphorescence and thermo- 

 luminescence (see Chapter IX) . Canton argued that his experi- 

 ment was definitely contrary to the wave theory and in favor of the 

 corpuscular theory of light: 



That a substance should either give light or not, when its parts are 

 agitated by the same degree of heat, according as it has, or has not been 

 exposed to light, for a few seconds of time more than six months before: 

 seems plainly to indicate a strong attraction between that substance and 

 the particles of light: 



Canton was no newcomer in study of luminescences for he had 

 previously carried out experiments on electroluminescence in 1753. 

 The same statement is true of Benjamin Wilson (1708-1788), a 

 Secretary of the Royal Society, perhaps best known for his electrical 

 experiments, which included a study of the electric discharge in 

 different pressures of gas (see Chapter VII) . Wilson's investigation 

 of phosphors began with a repetition of the experiments made by 

 Beccari some thirty years before, and then continued with the be- 



** Beccaria (1776) also studied the exciting action of electric sparks on phosphors. 



