338 History of Luminescence 



penetrates likewise into them; because it is attracted more powerfully 

 because of its greater density, and therefore expels again the light: The 

 more heat penetrates at once into the subtle pores, the quicker is the 

 light expelled; and the brighter shines the phosphorus.— Hence I collect 

 the cause, why these phosphori, after they are somewhat heated, do not 

 attract the light, as long as they still remain hot; because then the par- 

 ticular pores are filled up with heat. Humidity has the same effect. 



Most eighteenth-century combustion theories were based on the 

 presence of sulphur (the material, not the principle) an obviously 

 combustible body, in phosphors. Such was the attitude of Macquer 

 (1766, 1778) and also de Saussure (1792) , who spoke of " la com- 

 bustion a I'air libre du soufre ou du fois de soufre que le pierre 

 contenant." 



The same point of view is very definitely expressed in a paper 

 (1791) of Giovanni Marchetti (died 1817) professor of medicine 

 and chemistry at the University of Bologna. He had prepared phos- 

 phors from borax, sodium sulphate, and other compounds of sul- 

 phuric acid plus metals (Fe, Pb, Zn, etc.) , as well as vegetable ash, 

 all of which he thought contained a sulphur compound. Since many 

 sulphur-containing compounds do not burn in air, they were be- 

 lieved to contain a fluid which easily burned, called " gas hepa- 

 ticum " or " aer foeteus sulphuris," which smelt of rotten eggs and 

 made many metals black, i. e., he explained the phosphorescence by 

 the combustion of hydrogen sulphide. The phosphors were then 

 regarded as giving off their own light rather than absorbed light. 



Another writer who thought in terms of combustion was the 

 Rev. Mr. Morgan (1785) (not to be confused with Dr. William 

 Morgan, the student of electricity) . The Rev. Morgan's paper, 

 " Observations and Experiments on the Light of Bodies in a State 

 of Combustion," contained a section dealing with " Observations 

 on phosphoric light." He had noted the varied colors which salts 

 impart to flames, often accompanied by a " dense fume of unignited 

 particles," and thought that when light escapes from a burning body 

 blue comes first. To him this meant " that light, as an heterogeneous 

 body, is gradually decomposed during combustion . . . that the 

 indigo rays escape with the least heat and the red with the greatest." 

 Whereas Newton said " flame is a vapour heated red hot," Morgan 

 said " flame is an instance of combustion whose colour will be deter- 

 mined by the degree of decomposition which takes place." 



Morgan then referred to the various phosphorescent colors ob- 

 served by Mr. Wilson in calcined oyster shells exposed to light and 

 wrote: 



