194 ^^^^' Morgan's Obfervations and Experiments on 

 the colours varied according to the degree of heat. At 

 the bale of the flame, or where the heat was leafl:, the in- 

 digo, the violet, and a very fmall tinge of the blue and 

 green appeared. As I approached the vertex of the flame, 

 the rays which efcaped became more and more numerous 

 till 1 reached the top, when all the rays appeared in' the 

 prifm. It fhould be attended to^ that when the red rays firfl: 

 made their appearance, their quantity was fmall, and gradually 

 increafed as the eye in Its examination approached that part 

 v.liere the heat was greateih Mr. Melvill, when he made 

 fome of the preceding experiments, obferved, that the yellow 

 rays frequently efcaped in the greatefl: abundance j but this An- 

 gularity proceeded from feme circumftances which efcaped his 

 attention* In confequence of mixing acids or falts with the 

 burning fpirits, a very denfe fume of unignited particles arifes, 

 and before the rays of the burning body arrive at the perfora- 

 tion where the prifm catches them, they mufl pafs through a 

 medium which will abforb a great part of the indigo and the 

 violet. On the other hand, owing to the imperfedion of the 

 decompofltion, very few of the red rays are feparated from their 

 bails, and confequently the yellow and the orange rays are 

 thofe alone which pafs through the unburnt fmoke of the 

 flame. 



I would now proceed with obferving, that, befides the in- 

 creafe or decreafe of heat, there are other modes of retarding 

 or accelerating the combuftion of bodies, by which alfo may 

 be examined fome of the preceding illuftrations, 



I. A candle burns moft rapidly and brilliantly In dephlo- 

 gifticated air. 



2. The 



