the Light cf Bodies in a State of Combujlion. j 209 
£xp. xv. Place a fhell whilfl emitting its green rays on a 
warm 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 flow combaflion the following ob- 
jections may be oppofed. 
i°. If phofphoric fhells owe their light to this caufe, we 
m oft confider the word combufiion when applied to them as 
implying in its hgnification all thofe circumflaoces which are 
the nfual attendants of a body whilfl 0,11 fire. Amongfl other 
neceffary confequcnces in fuch a cafe, the increafe of heat muff 
increafe the decompoiition of the combuflible ; whereas we 
difco-vei an effedt the very oppofite 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. 
Befides, when a phofphoric fhell has been made very hot, and 
while it has continued fo, I have conveyed the mofr brilliant 
difcharge of a battery over it without effedh In other words, 
heat, or the very caufe which promotes combufiion in all other 
inflances, in this particular cafe puts an end to it. Mr. Wil- 
son, in his Treatile on Phofphori, has defended an experiment 
fimilar to the preceding. But the refult he 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 fufped rnyfelf of any inaccuracy. 2 0 . When bodies 
are waded by combufiion, they can never be made to re-affume 
the appearances which they previoufly difplayed. No power 
can give to allies the phenomena of a burning coal. But 
phofphoric bodies are very different in this refped ; for a fhell 
may be made to lofe all its light by expofure to heat, and again 
Yol.LXXV. Ee Ly 
