and on the Condition of the Sun's Surface. 91 



refrangibilityj are shells tte liglit of Nvhich is orange^ yellow, 

 green, blue, indigo, violet. When we look upon such a flame, 

 the rays issuing froui all the coloured strata are received by the 

 eye at once, and impress us with the sensation of white liglit. 



The differently coloured shells of which a flame thus consists, 

 may be easily parted out from one another, and demonstrated by 

 a prism. Their cause is the slower rate at which combustion 

 occurs at points more and more towards the interior. On the 

 outside, which we may say is iu contact with the air, the com- 

 bustion is most vigorous and complete, and hence the light there 

 emitted is violet ; but in the most interior portion of the shining 

 shell, resting upon the dark combustible matter, the atmospheric 

 air can hardly penetrate, or rather its oxygen is exhausted and 

 consumed. Between the exterior and interior surface, the burn- 

 ing is going on with an activity constantly declining, because the 

 interpenetratiou or supply of oxygen is gradually less and less. 



But besides this collection of coloured shells, constituting 

 what may be termed the actual flame, there is another region 

 exterior thereto, and to be distinguished both in its chemical 

 nature and in its optical relations. Chemically, it consists of the 

 products of combustion and of the unburnt residues of the air, 

 that is to say, carbonic acid, steam, and nitrogen. These are 

 all the time escaping out of the true flame, and envelope it as an 

 exterior cone or cloak. Optically, this portion differs from the 

 true flame in the circumstance, that it is shining as an incandes- 

 cent, ignited, but not a burning body. For physiological reasons, 

 into the detail of which it is not necessary here to go, the tint 

 of this exterior cloak seems to be a monochromatic yellow. 

 That, however, is to a considerable degree a deception, prismatic 

 examination proving that all the other colours are present, and 

 that the yellow merely exceeds the rest in force or intensity. 



A flame thus far maybe considered as offering three regions : 

 — 1st, a central nucleus, which is not luminous, and consists of 

 combustible vapour ; 2ndly, an intermediate portion, the true 

 flame, arising from the reaction of the air and the combustible 

 vapour, and being composed of a succession of superposed shells, 

 the interior being red, the exterior violet, and the intervening 

 ones coloured in the proper order of refrangibility ; the cause of 

 this difference of colour being the declining activity with which 

 the combustion goes on deeper and deeper in the flame. As to 

 temperature, the inner red shell cannot be less than 977° F., and 

 the exterior violet one probably more than 2500° V. 3rdly, 

 an envelope consisting of the products of combustion, exterior to 

 the true flame, shining simply as an incandescent body, and its 

 light for the most part overpowered by the brighter portion 

 within. 



By the aid of the facts thus presented, wc can easily explain 



