the Light of Bodies in a State of Combujlion. 295 



tlie point He 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 the wood be inlulated, and a flrong fpark 

 which ftrikes 011 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 all 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 diminishing the depth of the points, to give the 

 fpark the following fuccellion of colours. When they w^re 

 deepeft below the furface, 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 ftiil, the yellow ; 

 and fo on till, by making the fpark pafs through the wood very 

 near its furface, allthe 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 phicnomena 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 



brilliantlj 



