VOL. LXXV.] PHILOSOPHICAL TRANSACTIONS. 6(JQ 



By examining the flame of a common candle we may observe, that its lowest 

 extremities, or the part in which the black colour of the wick terminates, dis- 

 charges the least heat; and that, as the vertex of the flame is approached, a suc- 

 cessive order of parts is passed through, in which the lowest is continually adding 

 to the heat of what is just above it, till we come to the top of the flame, near 

 which all the heat is collected into a focus. At the lowest extremity however, 

 where the heat is inconsiderable, a blue colour may be always observed; and from 

 this appearance, among others, it may be safely concluded, that the blue rays are 

 some of those which escape from combustibles in an early period of their decom- 

 position ; and that if the decomposition could be examined in a period still more 

 early, the colour of their flame would be violet. By an a priori deduction of this 

 kind, I was led to watch the appearances of a candle more attentively ; whence I 

 found that to the external boundary of a common candle is annexed a filament of 

 light, which, if proper care be taken to prevent the escape of too much smoke, 

 will appear most beautifully coloured with the violet and indigo rays. To the 

 preceding instance of a common candle many facts may be added, which speak a 

 similar language. If sulphur or aether is burned, or any of those combustibles 

 whose vapour is kindled in a small degree of heat, a blue flame will appear, which, 

 if examined by the prism, will be found to consist of the violet, the indigo, the 

 blue, and sometimes a small quantity of the green rays. The best mode however, 

 of showing the escape of some rays by that degree of heat which will not separate 

 others till increased, is the following. Give a piece of brown paper a spherical 

 form, by "pressing it on any hard globular substance : gradually bring the paper, 

 thus formed, to that distance from the candle at which it will begin to take fire . 

 in this case a beautiful blue flame may be seen, hanging as it were by the paper 

 till a hole is made in it, when the flame, owing to the increased action of the air 

 on all parts of it, becomes white, though the edges still continue of a blue or 

 violet colour. As a confirmation of what is here concluded from the preceding 

 facts, it may be observed, that the very flame which, when exposed to a certain 

 degree of heat, emitted the most refrangible rays only, will, if exposed to a greater 

 degree of heat, emit such as are less refrangible. The flames of sulphur, spirits 

 of wine, &c. when suddenly exposed to the heat of a reverberatory, change their 

 blue appearance for that which is perfectly white. But to gain a more striking 

 diversity of this fact, I adopted Mr. Melvill's mode of examining bodies while on 

 fire. I darkened my room, and placed between my eye and the combustible a 

 sheet of pasteboard, in the centre of which I made a small perforation. As the 

 lip-ht of the burning body escaped through this perforation, I examined it with a 

 prism, and observed the following appearances. When the spirits of wine were 

 set on fire, all the rays appeared in the perforation; but the violet, the blue, and 



