306 History of Luminescence 



thus giving merely the appearance of a glowing coal.- However, 

 the diamond described by Cellini in Due Trattati dell' Ori fie era 

 (1568) , which would shine after exposure to light must be authen- 

 tic, since certain varieties of diamonds do possess this property, a 

 fact established beyond question by Robert Boyle in 1662. Cellini's 

 diamond is perhaps the first recognized natural phosphor. 



In addition to jewels, certain natural phosphorescent minerals 

 may have been known before the discovery of prepared phosphors. 

 About 1600, Jan Baptista van Helmont (1577-1644) had in his 

 possession a stone or a flint (silicem) which would receive the light, 

 and preserve it, visible in the dark for some time, but no one knew 

 how it was prepared and " almost the memory of it has died with 

 the inventor," according to B. Wilson (1775) . A translation from 

 the Latin of van Helmont's statement from Magnum Oportet ^ is as 

 follows: 



Indeed I observe that if I expose a stone (silicem) to the air, when the 

 sun is high on the horizon, at least during two or three hours (and it 

 does not matter if the day is clear or if it is cloudy) , when after I move 

 this stone in a dark place, the stone retains the light of the sun, during 

 about the same period of time. This happened every time I repeated 

 the exposure to light above mentioned. . . . 



This account is certainly a straightforward description of phospho- 

 rescence. In addition to Wilson, Beccaria (1744) and Priestley 

 (1772) both mentioned van Helmont's flint stone. Heinrich (1811: 

 13) thought it might be a diamond or fluorspar, while Kayser (1908: 

 604) believed it to be fluorspar. The suggestion is most probable, 

 as certain varieties of this flint-like stone are phosphorescent, espe- 

 cially if warmed slightly by the heat of the hand (see Chapter IX) . 



The Bolognian Phosphor 



CASCARIOLO, GALILEO, AND LA GALEA 



Despite the tindoubted observations of natural phosphorescence 

 previously mentioned, the importance of the discovery of methods 

 for preparing artificial phosphors cannot be overestimated. This 

 technique was perfected between 1602 and 1604* by Vincenzo 



^ One of the best known books on jewels of the seventeenth century was Boeciiis de 

 Boot's (P1550-1632) , Geynmarum et lapidum historia (Ludguni Batavorum, 1647) , to 

 which was added the De gemtnis et lapidibus libri II, by Jean de Laet (1593-1649) , a 

 translator of Theophrastus. They repeated the story of carbuncles, etc., that " shine " 

 at night. See also John Hill (1746, 1774) . 



^Section 35, bearing the date 1600. Translated by B. Wilson (1775) . 



* Priestley's (1772:361) date, 1630, is undoubtedly a misprint. 



