340 History of Luminescence 



wrote ^^ to Joseph Priestley in 1776 that the Bolognian phosphor 

 made the air highly phlogistic (phlogisticated air = nitrogen) and 

 effected a quick and astonishing decrease in volume, but to obtain 

 this effect the phosphor must be a good one and the weather not 

 too cold. Kayser (1908: 623) has suggested that Volta observed 

 adsorption of some gas by the phosphor rather than combination 

 with oxygen. The experiment did not succeed well with Canton's 

 phosphor, and the phosphorus of urine " diminished the air rather 

 slowly." It is now known that oxygen is not necessary for phospho- 

 rescence, and phosphorescence is not a burning. 



The Effect of Ultraviolet and Infrared Rays 



In Benjamin Wilson's time the spectrum of sunlight had not been 

 divided into the infrared (heat) , the visible and the ultraviolet 

 (actinic) rays, nor were the differences in spectral distribution of 

 various light sources appreciated. That waves existed beyond the 

 violet appears to have been discovered in 1801 by Johann Wilhelm 

 Ritter (1776-1810), a physician of Munich. He found that this 

 ultraviolet region of the spectrum would blacken silver chloride, a 

 phenomenon observed to occur in white light by the German physi- 

 cian, Johann Heinrich Schultze *^ in 1727. That blackening is most 

 marked in the blue and violet region ** was an observation of K. W. 

 Scheele, in 1777. 



The possibility that ultraviolet rays might excite a phosphor was 

 suggested by Humphry Davy to Sir Henry Charles Englefield (1752- 

 1822) , who found (1802) that Canton's phosphor was excited much 

 more by the blue than the red end of the solar spectrum. Englefield 

 wrote: 



There was great reason to suspect that this power, like that of blackening 

 silver nitrate, extended beyond the visible blue ray; but our apparatus 

 was not prepared for the more delicate part of these experiments, which 

 are only mentioned with a view to exciting further research on this very 

 interesting subject and of giving Mr. Davy the credit due him for having 

 thought of the experiment. 



Like many important discoveries, the effect of ultraviolet and 



*^ In Priestley, Experiments and observations on different kinds of air 3, Appendix: 

 381-382, 1777. 



*^ It is said that Schultze discovered the blackening in an attempt to prepare a 

 phosphor by treating chalk with nitric acid. The nitric acid happened to have silver 

 dissolved in it and the resultant silver salt blackened in sunlight. 



** It was not until 1873 that H. W. Vogel discovered photo-sensitization, the fact 

 that certain dyes added to silver salts would make them sensitive to green and yellow 

 light. 



