Explosive Gases and Gaseous Mixtures. 333 



lene propagates itself throughout the gas. About the path of 

 the spark there are sufficiently numerous encounters of mole- 

 cules with a velocity adequate to cause decomposition, and the 

 heat resulting is not lost by radiation before other molecules 

 of acetylene are decomposed. Thus the change continues 

 until only solid carbon and stable molecules of hydrogen 

 remain. A proposed method for the commercial production 

 of lampblack depends upon this fact. When, however, a 

 spark passes through the gas at common pressure, the change 

 occurs only in the path of the spark. Here the impacts of 

 molecules with a velocity adequate to cause decomposition are 

 more infrequent than in the denser gas — too infrequent, as the 

 result shows, to propagate the change. The heat about the 

 path of the spark is dissipated by radiation before it decom- 

 poses neighboring molecules. The infrequency of impacts 

 allows sufficient time for radiation, and for the molecules to 

 assume a stable condition. It is also probable that some more 

 stable molecules, such as C 6 H 6 , are formed. If acetylene issu- 

 ing from a tube is lighted, the decomposition does not extend 

 into the tube for the same reason that it does not extend 

 through the gas when sparked at ordinary pressure. 



The deportment of acetylene under high pressure, when sub- 

 jected to small sparks (p. 324) is similar to that of detonating 

 gas.* Condensation products are formed in one case and 

 water in the other. The minute spark imparts to the mole- 

 cules of acetylene energy adequate to effect combination, and 

 as the spark is visible we assume that the portion of the gas 

 which is glowing has a temperature at which acetylene decom- 

 poses when larger quantities are heated. Thus the infrequency 

 ci impacts of molecules of high velocity is the real reason why 

 feeble sparks do not start an explosion in dense acetylene. 



The non-explosion of cyanogen (p. 325) under high pres- 

 sure when sparked, although it is more endothermic than acety- 

 lene, is doubtless connected with the fact that it requires a 

 higher temperature for its decomposition than the latter ; or it 

 may be that the union of CN with CN" is exothermic, while 

 the radical itself is endothermic. Nitrous oxide and nitric 

 oxide are stable at high temperatures and these also do not 

 decompose with explosive violence when sparked under pres- 

 sure. It is probable that all three of these endothermic gases 

 would explode if sparked under pressure and at a temperature 

 approaching that at which they dissociate. 



*This Journal, iv, 51. 



