136 History of Luminescence 



light, and fire. The book on fire, Dissertationes Physicae de Igne 

 (Francofurti et Lipsiae, 1688) , is best known (see title page in figure 

 12) . In dissertatio 12 of De Igne, entitled, " De Luce Ignis," some 

 seven pages are devoted to glowing eyes of cats, the light of the sea, 

 luminous wood, mushrooms, the Nyctegretus of Pliny, luminous 

 insects, the carbuncle, and the four kinds of phosphorus, viz., Bono- 

 niensis, hermeticus, smaragdinus, and fulgurans. He was one of 

 those who had a single explanation for all the luminescences— salt. 

 Casati discussed these luminescences in the form of a conversation 

 between three persons, one of whom was Dandulus. Each con- 

 tributed some statement from previous writers or an opinion of 

 his own. It was generally agreed that the glow of the eye was a real 

 light emission. Dandulus, however, held that some light might be 

 subjective (from the spirits) , as the " sparks " observed when a 

 person coughs hard,^^ but sometimes light must be attributed to 

 salts (salibus) : *^° 



If you cut in the dark a gourd seasoned with much sugar and covered 

 with a hard crust, you will see sparks flashing. When I saw this unex- 

 pectedly at an early age, I was quite overcome. But this is nothing 

 spectacular, for the sea water itself when stroked by oars at night some- 

 times sends forth sparks, as I have heard from trustworthy persons who 

 have sailed in triremes. 



Dandulus held that the sea is saltier and more luminous in the 

 tropics (Brazil for example) than toward the poles, and continued: ®° 



I note the combination of a great amount of salt and much light in the 

 same water. And it seems that this argument cannot be easily dismissed, 

 since the wood of rotten oaks shines at night. For oakwood is of extra- 

 ordinary weight, not so much from the admixture of earth but also from 

 its abundance of salt. . . . On the other hand, light wood, such as the 

 poplar tree, which contains but little salt, does not shine at night. 

 Agaricus, the luminous fungus, also contains salt, for did not Pliny say 

 that Chiefly the acorn-producing trees of the Gauls carry agaricus . . . 

 [which] shines at night. 



The close relationship between light and salt was also proved by 

 the Bononian stone, for when the sulphur is removed by calcina- 

 tion (p. 351) , " what is left is a very strong and biting salt . . . and 

 with this salt light combines easily and, imbued with it, it shines in 

 a dark place." Not so much was known of the other phosphors, 

 particularly of the element phosphorus, which in the liquid form 

 (dissolved in oil) could be rubbed on bodies and (p. 354) , 



^* These " sparks " are actually a phenomenon connected with light refracted by 

 white blood corpuscles moving in blood vessels over the retina. 

 «" P. Casati, De igne, 349-350, 1688. 



