the Light of Bodies in a State of Combufion, 2© 5 



the point He about onc-elghth of an inch below the furfac;-. 

 Let another pointed wire, which communicates with the ground^ 

 be forced in the fame manner into the lame wood, fo that 

 its point likewife may He about one-eighth of an inch below 

 the furface, and about two inches dlfiant from the point of 

 the firft wire. Let the wood be infulated, and a ftrong Ipark 

 which il:rikes on the metaUic ball will force its paliage 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 all the rays hut 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- 

 creafnig or diminifliing the depth of the points, to give the 

 fpark the following fucceflion of colours. When they were 

 deepeft below the lurface, the red only came to the eye through 

 a prifm. 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 

 fiarface of foft deal wood^ the red, the orange, and the yel- 

 low rays will appear as the fpark pafles through it. But whea 

 the points are at an equal depth in a harder piece of wood (fuch. 

 as box) the yellow, and perhaps the orange, will difappear. Aa. 

 a farther proof that the phifinomena 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, 



hrilUaiitly 



