the Light of Bodies in a State 6f Comhujflon, 20^ 



EXP. XV. Place a fhell whilfl emitting its green rays on a 

 tvarm fhovel, and the appearance of the fhell will be foon 

 changed into that of a yellow mixed with red. To Mr. Wil- 

 son's theory, however, of (low combuftion the following ob- 

 -je£lions may be oppofed. 



1°. If phofphoric fhells owe their light to this caufe, wc 

 muft confider the word combuftion when applied to them as 

 implying in its fignification all thofe circumftances which are 

 •the ufual attendants of a body whilfl on fire. Amongfl other 

 'iiecefTary confequences in fuch a cafe, the increafe of heat mufl 

 "increafe the decompofition of the combuflible ; whereas wc 

 difcover an effect 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 lofing the power of be- 

 coming phofphoric again when it has been fufficiently cooled. 

 Beiides, when a phofphoric fhell has been made very hot, and 

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

 difcharge of a battery over it without efFe61:. In other words, 

 lieat, or the very caufe which promotes combuflion in all other 

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

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

 iimilar to the preceding. But the refult he mentions is diiferent 

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

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

 Juflly fufpe(5l myfelf of any inaccuracy, z'". When bodies 

 are wafted by combuftion, they can never be made to re-aflume 

 the appearances which they previoufly difplayed. No power 

 can give to afhes the phaenomena of a burning coal. But 

 phofphoric bodies are very different in this refpect ; for a fhell 

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

 Vol. LXXV, E e mss 



