Reduced Pressures by Impulsive Electric Sparks. 355 



That of ethane is a definite increase of inflammability, that 

 of propane is not more than an indication of its presence. 

 The propane curve is otherwise an approach to the hyper- 

 bolic form associated with ignition by condenser discharge 

 at low pressures. 



Comparing the hydrogen and methane curves in view of 

 the above, the commencement of the "low stage, which is 

 probably that of ionization by collision rising suddenly 

 to saturation, is seen to be unfavourable to the ignition of 

 methane, favourable to that of hydrogen. Now according 

 to Bone's accepted theory*, the combustion of methane pro- 

 ceeds by the successive addition of atoms of oxygen to the 

 hydrogen of the molecule. In the normal high-temperature 

 arc or spark oxygen is dissociated and atomic oxygen is 

 free to combine and start self-ignition. As the pressure is 

 reduced and the spaiks become cooler there is less of this 

 action and in the glow discharge high-temperature dissocia- 

 tion will almost cease. There are then present molecules and 

 free electrons, a state which is perhaps less favourable to 

 combination than when oxygen is atomic. 



On the other hand, Bone has shown that "in explosion 

 flames hydrogen is directly burnt to steam as the result of 

 trimolecular impacts," and a phase in which there is ionization 

 of molecules of gas by collision with electrons without disso- 

 ciation is favourable to such an action. It has been shown 

 elsewhere that as the gases rise in the paraffin series ignition, 

 which became suddenly more difficult in the step from 

 hydrogen to methane, becomes easier, and that pentane 

 approaches hydrogen in sensitiveness and type. Ethane and 

 propane are intermediate, and it is not difficult to follow the 

 transition from the methane curve of fig. 2, by that of ethane 

 to that of propane, falling in towards the origin and showing, 

 though to a much smaller extent, the fluctuation so marked 

 in hydrogen. 



9. Carbon Monoxide. 



The curious cusp in fig. 5 has been found in carbon 

 monoxide with more than one kind of ignition. It is a 

 sudden but not permanent increase of difficulty of ignition 

 occurring at much the same pressure as the change first 

 discussed. It would appear to depend more upon the varia- 

 tion of spark intensity, or temperature of the pole at the 



* W. A. Bone aiid other?, " Gaseous Combustion at High Pressures," 

 Phil. Trans, ser. A, vol. 215. p. 304. 



