Phosphorescence 339 



I£ with Mr. Wilson we suppose, that these shells are in a state of slow 

 combustion, may we not conclude, that some are just beginning to burn, 

 and therefore, agreeably to what I have observed on combustible bodies, 

 emitting the most refrangible rays; whilst others are in a more advanced 

 state of combustion, and therefore emitting the least refrangible. If this 

 conclusion be right, the shells which are emitting the purple, or the 

 green, must still retain the yellow, the orange, and the red, which will 

 also make their appearance as soon as the combustion is sufficiently 

 increased. 



In line with this theory he reported that Wilson's oyster shells, which 

 emitted a green phosphorescence, would become yellow to red lumi- 

 nescent when warmed. 



However, Morgan found some objections to his own view. The 

 most important was the fact that a phosphoric body " never fails to 

 lose its light entirely in a certain degree of heat, without losing the 

 power of becoming phosphoric again when it has been sufficiently 

 cooled." 



Connected with the problem of combustion, was the identifica- 

 tion and study of gases, which received much attention in the latter 

 part of the eighteenth century, largely owing to the paper of Caven- 

 dish on gases in the Phil. Trans, for 1766, and to Priestley's book. 

 Experiments and Observations on Different Kinds of Air (1774). 

 Phosphors, the element phosphorus, shining wood, and shining meat 

 were all subjected to the action of various gases, but the impurity of 

 eighteenth-century samples of these gases led to conflicting results 

 and endless controversy. Compared to the numerous studies on 

 shining wood and luminous meat, experiments on phosphors were 

 definitely neglected. Moreover, the conclusions drawn from the most 

 elaborate series of experiments were quite incorrect. 



In 1786 Count Carlo Luigi di Morozzo (1744-1804), an Italian 

 physicist of Turin, studied the behavior in different gases, finding 

 that the Bolognian phosphor only luminesced well in dephlogis- 

 ticated air (oxygen) , like all combustible bodies, and also that the 

 air itself was changed by its presence. In fact, he claimed that the 

 presence of the phosphor in aria fissa (CO2) , aria nitrosa (nitrous 

 air) and aria inflammabile (hydrogen ?) made these gases respirable 

 for a short time. The conclusions are extraordinary and could not 

 be explained by Count Morozzo, although he claimed that he had 

 " collected many facts and discovered new properties concerning 

 gases, which one day may contribute to a reasonable theory in this 

 new branch of science." 



Volta had apparently obtained similar results earlier, for he 



