|MadSci Network: Chemistry|
Heat travels upward due to its convective currents. Fire, having little or no mass in itself, travels the path of the convected heat currents, hence the shape of the flame. It doeson't get any simpler than that. Remember, however, that fire is defined as a self sustaining process of combustion which generates heat, light, gases and smoke. The only one of these things which is truly massless is heat, Light has a very small amount of mass according to the theory of relativity, albeit immeasureable, it is still implied. Thanks for the question. Donald e duggan
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