1894.] The Action of Heat upon Ethylene. 91 



at the same time losing its property of forming an oily liquid with 

 chlorine. 



These experiments were afterwards repeated by Fourcroy, Hecht, 

 and Vauquelin,* who showed that when heated, ethylene yields 

 hydrogen with deposition of carbon, whilst in 1805 William Henryf 

 showed that ethylene was formed during the destructive distillation 

 of organic bodies, and that on farther heating the gas, other changes 

 were observed, and the gas was eventually converted into carbon and 

 hydrogen. The deposition of carbon was also noticed later by Quet,J who 

 on passing sparks through ethylene found that carbon was deposited, 

 and formed a bridge between the poles used for the discharge, whilst 

 Dalton showed by the continuous action of the electric spark that 

 ethylene yielded double its own volume of hydrogen, carbon being 

 deposited. 



Marchand came to the conclusion that at a red heat this gas splits 

 up into methane and carbon, but at a white heat into carbon and 

 nearly pure hydrogen, whilst Magnus, in 1847, made the important 

 observation that on leading ethylene through a red-hot tube a con- 

 traction in volume followed ; the residual gas consisted of methane, 

 hydrogen, and unchanged ethylene, whilst carbon was deposited, and 

 fluid and even solid hydrocarbons were obtained. 



In 1860 H. Buff and A. W. Hofmann|| published a paper on the 

 *' Dissociation of Gaseous Compounds on Heating by Electricity." 



They found that when a platinum spiral is heated by the galvanic 

 current in pure ethylene there is at once a visible separation of 

 carbon, which covers the sides of the tubes with a black deposit, 

 whilst hardly any expansion in the volume of the gas takes place, 

 from which they assume that the ethylene has split up into methane 

 and carbon. 



If the action on the gas, due to the incandescent platinum wire, is 

 allowed to. continue, then an increased amount of the gas undergoes 

 dissociation, and soon after the separation of carbon commenced, they 

 observed an expansion which is rapid at first, and in ten minutes 

 reaches a maximum. Similar phenomena were noticed with the spark 

 current ; at first the spark had a pale reddish tint which gradually 

 turned to violet, immediate separation of carbon taking place, the spark 

 being frequently stopped by scales of carbon which formed a bridge 

 between the poles. They found that under these conditions the volume 

 of gas expands very rapidly at first but afterwards more slowly, 

 and that after twenty to twenty-five minutes, the point of maxi- 



* Gilbert's ' Annalen,' TO!. 2, p. 210. 



t 'Nicholson's Journal,' 1805. 



J ' Comptes Eendus,' vol. 42, p. 903. 



' J. f lir prakt. Chem.,' TO!. 26, p. 478. 



|| Liebig's ' Annalen der Chemie,' vol. 113, p. 119. 



