the Light of Bodies in a State of Combujiion . 2©^ 
the point lie about one-eighth of an inch below the furface. 
Let another pointed wire, which communicates with the ground, 
be forced in the fame manner into the fame wood, fo that 
its point likewife may lie about one-eighth of an inch below 
the furface, and about two inches diftant from the point of 
the firft wire. Let tiie wood be infulated, , and a ftrong fpark 
which hrikes on the metallic ball will force- its paflage through 
the interval of wood which lies between the points, and appear 
as red as blood. To prove that this appearance depends on the 
wood’s abforption of ail the rays but the red, I would obferve, 
that the greater the depth of the points is below the furface, the 
lefs mixed are the red rays. I have been able fometimes, by in- 
creafing or dimmilhing the depth of the points, to give the 
fpark the following fucceffion of colours. When they were 
deepefl below the furface, the red only came to the eye through 
a prifmn When they were raifed a little nearer the furface, 
the red and orange appeared. When nearer ftill, the yellow ; 
and fo on till, by making the fpark pafs through the wood very- 
near its furface, all the rays were at length able to reach the 
eye. If the points be only one-eighth of an inch below the 
furface of foft deal wood, the red, the orange, and the yel- 
low rays will appear as the fpark pafles through it. But when 
the points are at an equal depth in a harder piece of wood (fuch 
as box) the yellow, and perhaps the orange, will difappear. As 
a farther proof that the phenomena I am defcribing are owing to 
the interpofition of the wood, as a medium which abforbs fome 
of the rays and fuffers others to efcape., it may be obferved, that 
when the fpark ftrikes very brilliantly on one fide of the piece 
of deal, on the other fide it will appear very red. In like man- 
ner a red appearance may be given to a fpark which ftrikes 
brilliantly 
