VOL LXXV.J PHILOSOPHICAL TRANSACTIONS. 67 1 



most refrangible rays escape in the greatest abundance ; but of these rays, the 

 green escape most plentifully through the unignitetl vapour and that portion of 

 the atmosphere which separates the eye from the flame. The peculiarity which 

 I have now endeavoured to account for may be observed in the greatest perfection 

 in brass foundries : the heat in this instance, though very strong, is scarcely 

 adequate to the decomposition of the metallic vapour which escapes from the 

 melted brass. A very singular flame therefore appears to the eye ; for while its 

 edges are green, its body is such as to give the objects around a very pallid or 

 ghastly appearance, which is the consequence of its wanting that portion of red 

 rays which is necessary to make a perfect white. 



The most singular phenomenon attending a burning body, is perhaps the red 

 appearance it assumes in its last stage of combustion. The preceding facts and 

 observations may help us to explain it. 1. After a body has continued to burn 

 for some time, its external surface is to be regarded as having lost a great por- 

 tion, if not the whole, of those rays which the first application of heat was able 

 to separate. But these rays were the indigo, the violet, the blue, and perhaps 

 the green. Nothing therefore will remain to be separated, but the yellow, the 

 orange, and the red. Consequently, the combustion of the body, in its last 

 state of decomposition, can assume no other than a reddish appearance. But 

 1. Let us consider the external surface of the combustible as annexed to an 

 inner surface, which may be partly, but not so perfectly decomposed as itself: for 

 the violence of the heat will be found to lessen in its effects the nearer it ap- 

 proaches to the centre of the substance exposed to it. Hence we are to consider 

 the parts just covered by the external surface as having lost less of their com- 

 ponent light than the external surface itself. Or the former may retain the green 

 rays when the latter has lost both indigo, violet, blue, and green. 3. Those 

 parts which are nearer the centre of the body than either of the preceding, 

 must as they are farther from the greatest violence of the heat, have lost pro- 

 portionally fewer of their rays. Or while the more external parts may have 

 lost all but the red, these may have lost only the indigo and violet. 4. The 

 most central parts may be unaffected by the heat ; and whenever the fire does 

 reach these parts, they will immediately discharge their indigo rays, and be de- 

 composed in the gradual manner already described. A piece of rotten wood, 

 while burning, will exemplify and confirm the preceding illustration. When 

 influenced by the external air only, if examined through a prism, no rays will 

 be found to escape but the orange and the red. By blowing on the burning 

 wood with a pair of bellows, the combustion, being increased, will affect those 

 internal parts of the body which were not acted on before. These parts there- 

 fore will begin to lose their light, and a prism will show the green, the blue, the 

 violet, and indigo, all appearing in succession. Appearances similar to the pre- 



