196 History of Luminescence 



professor of chemistry at the University of Pavia, held (1797) that 

 light existed in three forms in bodies. First, there was chemically 

 bound light which is freed on heating because of " its affinity for 

 caloric," as in fluorspar, various calyces, corrosive sublimate, sugar, 

 camphor, turpentine, oils, and fats. All these substances shine with 

 great vivacity on a hot plate, either as a solid or in the form of 

 vapor and Brugnatelli held that air was not necessary for the light. 

 Second, there was mechanically held but invisible light, freed by 

 approximation of the parts of materials, thereby pressing out the 

 light, as in the mercurial phosphor, " vitriolated tartar " on crystal- 

 lization, in the phosphorescence of sea water, pounding or scraping 

 sugar, striking cream of tartar, alum or borax, knocking quartz 

 pebbles together, the eye when struck, and in certain plants. Third, 

 there was mechanically held but visible light, as with the Bolognian 

 and Baldeweinian phosphors, diamonds and other precious stones, 

 the eyes of the cat and hyena, putrified fish, rotten wood, the body 

 of the glowworm and other phosphorescent animals. Apart from 

 the fact that many of the above luminescences are not true light 

 emissions, a more illogical scheme is hard to imagine, but it may 

 serve to emphasize the advance destined to take place later in the 

 century. 



HUMPHRY DAVY 



Another distinguished scientist of the early nineteenth century, 

 Humphry Davy (1778-1829) , also thought of light as a substance 

 capable of combination. Despite his broad interest in chemistry 

 and physics and his eminence as an electrochemist, Davy never made 

 any concerted study of luminescences. However, in the Syllabus 

 of a Course of Lectures, delivered at the Royal Institution in 1802, 

 when he was only twenty-four years old, Davy (1799, 1803) men- 

 tioned ^ methods for the artificial production of light, which very 

 clearly indicate that he recognized phosphorescence, thermolumines- 

 cence, triboluminescence, chemiluminescence, and bioluminescence 

 as distinct categories. He later (1822) studied electroluminescence 

 in some detail. 



Davy spoke " Of Phosphorescent Bodies " as follows: 



Certain bodies, (solar phosphori) after being for some time exposed 

 at a high temperature to light, continue luminous for a considerable 

 length of time after this exposure. Such are many preparations of lime, 

 the bolognian stone, Sec. This phaenomenon is in some measure analogous 

 to the ignition of incombustible bodies. 



Light, it appears, is only susceptible of combining, and of remaining 



1 opera 2. Early miscellaneous papers, 1799-1805, 33-35, London, 1839. 



