ibe "Ltght of Eodks in u State of Comb ujl ton, ■20.9 



e:XP. XV. Place a fliell vvhllfl emitting its green rays on a 



warm fhovel, and the appearance of the fhell will be foou 



cl^anfred into that of a veilow mixed with red. To Mr. Wil- 



-son's theory, hou^ever, of (low combuftion the following ob- 



j-e6lions may be oppoled. 



i''. If phofphoric (hells owe their light to this caufe, wc 

 muft confider the word combuftion when applied to them as 

 implying in its iignilication all thofe circumftances which are 

 the ufual attendants of a body whilft on fire. Amongfl: other 

 iieceirary confequences in fuch a cafe, the increafe of heat muft 

 increalc the decompofition of the combuftible ; whereas we 

 •difcover an etl^^t the very oppolite to this in the appearance of 

 a phofphoric body, which never fails to lofe its light entirely 

 in a certain degree of heat, without lohng the power of be- 

 ^coming phofphoric again when it has been fufficiently cooled. 

 Befides, when a phofphoric ihell has been made very hot, and 

 while it has continued fo, I have conveyed the moft brilliant 

 difcharge of a battery over it without efFe(^. In other words, 

 heat, or the very caufe which promotes combuftion in all other 

 inftances, in this particular cafe puts an end to it. Mr. Wil- 

 son, in his Treatife on Phofphori, has defcribed an experiment 

 fimilar to the preceding. But the refulthe mentions is different 

 from that here mentioned. However, from a regard to his 

 authority, I have fo frequently repeated my trials that I cannot 

 juftly fufpe<5l myfelf of any inaccuracy. 2\ When bodies 

 are wafled by combuftion, they can never be made to re-affume 

 the appearances which they previoufly difplayed. No power 

 can give to alhes the phenomena of a burning coal. But 

 phofphoric bodies are very different in this refpe<5t ; for a fhell 

 may be made to lofe all its light by expofure to heat, and again 

 . Vol. LXXV. E e may 



