THE HYDROCARBONS AND THEIR DERIVATIVES. FLAME 355 



series (p. 348), with the formula dvHss.COOH, made from fat). 

 When it burns, the whole phenomenon is vastly more complicated 

 than the burning of hydrogen. The following are some of the 

 stages in the process, which is operated by the flame's own heat. 

 To start with, the wax is melted and ascends the wick by capillary 

 action. This is merely a physical phenomenon. Then the 

 chemical changes begin. (1) The melted compounds of carbon 

 are decomposed by the heat (cracked, p. 346), being turned into 

 more volatile compounds and gases which occupy the central 

 hollow of the flame. (2) The compounds forming the gases and 

 vapors are further decomposed at a white heat, giving free carbon 

 and hydrogen (p. 347). (3) All the materials finally reach 

 a sufficient supply of oxygen and are burned to water and 

 carbon dioxide. There are thus three chemical changes, each of 

 which takes place in a definite region that can be 

 observed by the eye (Fig. 89). The formation of 

 the gases from the melted wax (without gas, there 

 would be no flame) takes place in the dark central 

 region where there is no oxygen. The carbon is set 

 free and glows brilliantly in the luminous cone that 

 surrounds the gas and extends far above it. The 



final combustion occurs in a fainter cone of flame 



,, T T . FIG. 89 



covering the whole exterior. 



That there is unburnt gas (produced by decomposition of the 

 wax) in the center is easily shown by inserting a narrow tube, 

 through which some of the gas will ascend. The free carbon in 

 the luminous zone will show its presence by blackening a cold 

 dish placed across the flame. 



Lampblack. When an iron vessel, cooled by a stream of 

 water circulating through it, is suspended in the luminous flame of 

 natural gas or burning petroleum, the carbon (soot) is deposited 

 on the vessel. By rotating the latter, the soot can be continu- 

 ously scraped off by a stationary piece of metal. The product, 



