352 SMITH'S INTERMEDIATE CHEMISTRY 



we obtain not only the heat due to the combustion (p. 162) of 

 the carbon to carbon dioxide (4 X 96,900 cal.) and of the hydrogen 

 to water (116,200 cal.) but also the heat due to the decomposition 

 of the gas (2 X 58,100 cal.). The temperature of the flame is, 

 therefore, the highest that can be reached by the combustion of 

 any easily obtainable gaseous mixture. The oxy-acetylene flame, 

 produced by means of a suitable burner (Fig. 29, p. 56), the gases 

 being furnished from small, portable tanks, is now used for cutting 

 metals. Such a flame will melt its way through a 6-inch shaft 

 of steel, or a heavy steel plate several feet wide, in less than one 

 minute, cutting the object in two. Steel buildings have been taken 

 apart rapidly by this device. 



Blau gas and oil gas, mixtures of hydrocarbons made by " crack- 

 ing " (see p. 346) heavy oils, 'are now largely displacing acetylene 

 for uses like those just mentioned. They give flames which are 

 almost as effective, and are more easily controlled. Even the 

 oxy-hydrogen torch is remarkably efficient, when applied to the 

 same purposes. 



Benzene C 6 H 6 . This is the first member of the aromatic 

 hydrocarbon series. It may by synthesized by heating acetylene 

 in a closed vessel at a moderately low temperature: 

 3C 2 H 2 -> C 6 H 6 . 



In practice it is obtained, with many of its valuable derivatives, 

 as a by-product in the production of coke (p. 424). 



The graphic formula of benzene is represented as a closed 

 ring structure: 



H 



A 



H-C C-H 



II I 

 H-C C-H 



I 

 H 



