the Light of Bodies ia a State of Comhuftion, i^y 



gradual manner which I have ah'eady defcribed. A piece of 

 rotten wood, whilft burning, will exemplify and confirm the 

 preceding illuftration. When influenced by the external air 

 only, if examined through a prifm, no rays will be found to 

 efcape but the orange and the red. By blowing upon the 

 burning wood with a pair of bellows, the combuflion, 

 being increafed, will affect thofe internal parts of the body 

 which were not afted upon before. Thefe parts, therefore, 

 will begin to lofe their light, and a prifm wall fhew the 

 green, the blue, the violet, and indigo, all appearing in fuc- 

 cefiion. Appearances fimilar to the preceding may be ob- 

 ferved in a common kitchen fire. When it is falnteil, its 

 colour is mod: red, the other rays having been emitted, and 

 the combuftion at a {land ; but by blowing upon it in this 

 ftate, its brightnefs will be increafed, and more and more of 

 the rays which are yielded by the internal parts of the bgdy 

 will come to the eye, till at length, by continuing to blow, the 

 combufllon will be made fo complete as to yield all the rays, 

 or to make it appear perfectly white. 



Many are the varieties difcoverable in the flames and in the 

 appearances of fixed burning bodies to which the preceding 

 obfervations may be applied ; but, to avoid unneceflary amplifi- 

 cation I will take notice only of what appears to me an imper- 

 fe£lion in Sir Isaac Newton's definition of flame. He conr 

 jedures, that it may be a vapour heated red-hot. I think I 

 Ihould rather fay, that flame is an inflance of combuflion 

 whofe colour will be determined by the degree of decompofi- 

 tion which takes place. If it be very imperfedl, the moft 

 refrangible rays only will appear. If it be very perfect, all 

 the rays will appear, and its flame will- be brilliant in 



proportloii 



