3 94 ^^^' Morgah's Ohfe^vaitons and Expenmenis on 

 the colours varied according to the degree of heat. At 

 the bafe of the flame, or where the heat was leaft, the in- 

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

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

 the rays which efcaped became mors and more numerous 

 till. I reached the top, when all the rays appeared in the 

 prilra. It fhould be attended to, that when the red rays firft 

 made their appearance, their quantity was fmall, and gradually 

 increafed as the eye in its examination approached that part 

 where the heat was greatefl:. Mr. Melvill, w^hen he made 

 fome of the preceding experiments, obferved, that the yellow 

 rays frequently efcaped in the greateft abundance ; but this fin> 

 gularity proceeded from fome circumftanccs 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 prifra catches them, they muft pafs through a 

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

 violet. On the other hand, owing to the imperfection of tjie 

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

 bafis, and confequently the yellow and the orange rays are 

 thofe alone which pafs through the unburn t fmoke of the 

 flame. 



1 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- 

 g^ifticated air. 



z» The 



