NATURAL PHILOSOPHY. 129 



towards the interior. On the outside, which we may say is in contact with 

 the air, the combustion 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 burning is going on with an activity con- 

 stantly declining, because the interpenetration or supply of oxygen is gradu- 

 ally less and less. 



But, besides this collection of colored 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. Chem- 

 ically, it consists of the products of combustion and of the unburnt residue 

 of the air, that is to say, carbonic acid, steam, and nitrogen. These are all 

 the time escaping out of the true flame, and envelop it as an exterior cone 

 or cloak. Optically, this portion diifers from the true flame in the circum- 

 stance, that it is shining as an incandescent, 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 3~ellow. 

 That, however, is, to a considerable degree, a deception; prismatic examina- 

 tion proving that all the other colors are present, and that the yellow merely 

 exceeds the rest in force and intensity. 



A flame thus far may be considered as offering three regions : First, a 

 central nucleus which is not luminous, and consists of combustible vapor; 

 secondly, an intermediate portion, the true flame, arising from the reaction 

 of the air and the combustible vapor, and being composed of a succession of 

 superposed shells, the interior being red, the exterior violet, and the inter- 

 vening ones colored in the proper order of refrangibility; the cause of this 

 difference of color 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 2300 3 F. Thirdly, an envelop consisting of the products of com- 

 bustion, 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, we can easily explain the nature 

 of the other regional divisions, distinguishable in such a flame. There must 

 be a blue portion below; blue, because it consists of the most refrangible 

 rays, which issue forth in abundance, for there the exterior air is most copi- 

 ously and perfectly applied. At the upper end of the flame, particularly if 

 the wick be long and the supply of combustible matter abundant, the light 

 emitted is red ; for the products of combustion ascending past that part, make 

 it difficult for the exterior air to get access. 



Upon these principles we may also predict what color a flame will have 

 when we vary the circumstances of its burning. Tallow or wax, at temper- 

 atures greatly beneath their usually understood point of combustion, oxidize 

 with a pale violet phosphorescent light, quite perceptible, nevertheless, i-n a 

 dark room; and here the light is violet, for the supply of combustible mat- 

 ter is small, and that of the air abundant. The oxidation is therefore thoi'- 

 ough and prompt. For a like reason, sulphur, as we commonly see, burns 

 blue; but if a piece of it is thrown into nitrate of potash ignited in a cruci- 

 ble, the light yielded is of intolerable brilliancy, and absolutely white. Its 

 Avhiteness does not depend upon the physiological fact, that any color, if 

 it be intensely brilliant, will seem white to the eye; but it is optically white, 



