362 History of Luminescence 



ones can be considered. The older opinions are of special interest 

 in contrast with those of the present day. 



Previous to the electron concept, the explanations of phospho- 

 rescence passed through a series of beliefs, evolving as rapidly as 

 the general advance in knowledge of material phenomena would 

 permit. One of the oldest views was the sponge theory, that phos- 

 phorescence is connected with the absorption of light in a porous 

 material, to be later released, a conception introduced by La Galla 

 (1612) , held also by Liceti (1640) , Kircher (1641) , Schott (1656) , 

 Hoffmann (1700),Dufay (1735) , Scheele (1771) , Herbert (1773), 

 and Osann (1834). 



Another view, due to Montalbani (1634) regarded phosphores- 

 cence as akin to burning, because of the " sulphur " contained in 

 the phosphor, a view adopted by Homberg and in part by Lemery 

 (1698), by de Mairan (1717), at one time by Dufay (1726), by 

 Zanotti (1748) , Volta (1776), Macquer (1778), Morozzo (1786), 

 Marchetti (1791) , de Saussure (1792) in connection with thermo- 

 luminescence, and by Ritter (1803) . Dufay's views were somewhat 

 variable, as he appeared to rely at times on one and at times on the 

 other theory, while Beccari presented no special theory. Ideas of 

 combustion and chemical processes were not sufficiently clear to 

 classify certain beliefs, for example that of J. A. Le Due (1787) . 



According to the light absorption theory, calcination made the 

 stone more porous; according to the burning theory, calcination 

 brought the sulphur to the surface or freed it or " exalted " it, or 

 removed impurities. The observations of Zucchi (1652) , that a 

 phosphor emitted the same color of luminescence no matter what 

 the color of the light to which it was exposed, was definitely against 

 the light absorption theory but apparently had little effect on the 

 popularity of the absorption idea or the belief that light was a 

 material substance. 



Conceptions regarding the nature of light naturally influenced 

 the explanations of phosphorescence. A number of physicists pro- 

 posed vibration theories. Lemery (1698) spoke of vibrations of 

 small parts and Cohausen (1717) held that calcination modified 

 the Bolognian stone in such a way that it would hold the air (ether) 

 in a special modification which allowed it to vibrate, as when there 

 is light in the air around us. L. Euler (1777) , a proponent of the 

 wave theory of light, also suggested the vibration theory previously 

 mentioned, based on his concept of surface color, that when light 

 strikes a body, it sets up characteristic vibrations in the material 

 particles, practically resonance vibrations, which are transmitted to 

 the ether as the color of the object. A phosphor exposed to light 



