COMMON THINGS FIRE. 



of the surface of the flame. The arrows at the sides of the flame- 

 outside its surface represent the currents of the surrounding air 

 produced by the heat of the flame ; the oxygen, being attracted 

 by the intensely heated combustible vapour, approaches it, and, 

 by combining with it, sustains the combustion and produces 

 the light. The arrows above the flame indicate the current of 

 heated air, carbonic acid and aqueous vapour, the products of 

 the combustion which form an ascending column above the 

 flame. 



It will be apparent from what has been here stated, that the 

 luminous part of the flame is merely superficial. The vapour 

 within the surface of the flame not having yet come into contact 

 with the oxygen, and therefore not having entered into com- 

 bustion, cannot be luminous. The flame, therefore, so far as 

 relates to light, is hollow, or rather it is a column of combustible 

 vapour, the surface being the only part which burns, and there- 

 fore the only part which is luminous. As this vapour ascends 

 from the interior of the flame, it comes successively into contact 

 with the oxygen of the air, is burnt, and becomes luminous, the 

 column of light gradually contracting in diameter until it is 

 reduced to a point. The flame thus tapers to a point until all the 

 vapour produced by the boiling matter on the wick receives its due 

 complement of oxygen, and passes off. It speedily loses that 

 high temperature which renders it luminous, and the flame 

 terminates. 



37. In lamps of various construction, expedients are adopted 

 to increase the magnitude of the luminous surface of the flame, 

 and the intensity of the combustion. This is effected by modify- 

 ing the form and magnitude of the wick, by feeding it with an 

 abundant supply of oil, and by maintaining strong currents of 

 air at all parts of its surface to sustain the combustion. 



The most common form of wick used for lamps of strong illu- 

 minating power, is that of a hollow cylinder, varying from an 

 inch to three inches in circumference. This wick being attached 

 at its base to a small thin ring of metal is let down into the 

 reservoir of oil, through a space included between two concentric 

 tubes, one of which has a less diameter than the other, the space 

 between them being a little wider than the thickness of the wick. 

 The wick is from two and a half to three inches long, and descends 

 through this space between the tubes to a certain depth. This 

 space communicates with the reservoir of oil from which the oil is 

 forced up either by the action of a pump worked by a main spring, 

 through the intervention of wheelwork, as in the Carcel lamp, or 

 by the more direct action of a strong spiral spring as in the 

 Moderator lamp, or by the pressure of oil contained in a reservoir 

 206 



