28 Dr. Karl Heumann's Contributions to 



and the upper flame may be still farther increased by cooling 

 the combustible vapours. The following experiment is in- 

 structive : — 



A glass tube, 8 to 10 millims. wide and about 10 centims. 

 long, is fastened vertically in the middle of the slit in the 

 metallic cap surrounding the wick of a lighted petroleum-lamp. 

 By raising the wick a thick white vapour may be made to 

 issue from the upper orifice of the glass tube. If this vapour 

 be ignited, a small flame is produced, which plays above the 

 smoke at a distance of perhaps 10 centims. from the tube. 

 The products of combustion present above this flame are in- 

 visible, because the combustion is complete and the water 

 which is produced is dissipated by the heat evolved. 



If the column of visible vapour between the tube and the 

 small flame be carefully observed, it is seen to be rendered 

 transparent by the action of the heat radiated from the lower 

 flame, and finally to become ignited. In this way the fact 

 may be explained that the small flame does not rest directly 

 upon the visible column of vapour, but is separated from it 

 by a transparent space 1 or 2 millims. in extent. If the 

 glass tube in this experiment be replaced by one made of 

 platinum, and if this be heated, the small flame may be caused 

 to approach and finally to rest upon the orifice of the platinum 

 tube. 



The column of smoke which is seen to issue from a petro- 

 leum-lamp burning without the glass cylinder, is caused by 

 the cooling action of the metallic cap which surrounds the 

 wick. The flame-mantle impinges upon this metallic cap, is 

 thereby held back, and so is rendered unfit for propagating 

 the ignition upwards. The lower flame, being fed by air 

 entering from below, continues to burn, and produces new 

 gases and vapours from the oil-saturated wick, performing, 

 therefore, a part similar to that of the retort-fires in the 

 manufacture of coal-gas. 



As the metallic cap gets heated, the cooling action which it 

 exercises upon the stream of ascending vapours diminishes, 

 and the distance separating the upper flame from the lower is 

 decreased. If the metallic cap be heated by a Bunsen's lamp, 

 this distance becomes very small, and entirely disappears when 

 the cap begins to glow. If a cap already heated to redness 

 be placed upon a lighted and properly adjusted lamp, the 

 flame does not become separated at all. 



Everyday experience tells us that placing a glass cylinder 

 upon the lamp causes the two flames to unite. The diminished 

 supply of air brings about an elongation and curtailment in 

 the dimensions of the flame, whereby it no longer touches the 



