i 94 Mr. . Morgan's Obf creations and Experiments on 
the colours varied 1 according to the degree of heat; At 
the bale of the flame, or where the heat was leafh, the in- 
digo, the violet, and a very fmali 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 I reached the top, when all the rays appeared in the 
prifm. It fhould be attended to, that when the red rays firfb 
made their appearance, their quantity was fmali,. and gradually 
increafed as the eye in its examination approached that part 
where the heat was greatefh Mr. Mel vile, when he made 
fome of the preceding experiments, obferved, that the yellow 
rays frequently efcaped in the greateft abundance ; 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 mu A pafs through a 
medium which will abforb a great part of the indigo and the 
violet. On the other hand, owing to the imperfedlion of the 
decompoAtion, very fey of the red rays are feparated from their 
baAs, 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 observing* that, beAdes the lu- 
cre a fe or decreafe of heat,, there are other modes of retarding 
or accelerating the combuAion of bodies, by which alfo may 
be examined iome of the preceding illuftrations. 
i . A candle bums moft rapidly and brilliantly in dephlo- 
gifticated air. 
2. The 
