the Electric Discharge in Gases. 431 



turbance in some places may become so violent that in these 

 regions the ratio of the paired to the free times approaches 

 the value it has when the gas is about to be dissociated. 

 At this point any diminution of this ratio consequent upon 

 an increase in the intensity of the field will absorb a large 

 amount of energy; this energy must come from the electric 

 field; and we should thus get the phenomenon of the electric 

 discharge. The disturbance to which the gas is subjected 

 might very well account for the luminosity of the discharge; 

 whilst the heat produced by the recombination of the disso- 

 ciated gas, which would occur as soon as the disturbance due 

 to the electric field was withdrawn, would account for the heat 

 produced by the discharge. Only a very small amount of gas 

 would have to be decomposed in order to absorb the electrical 

 energy of the field. Taking the values of the electric force 

 necessary to produce discharge given by Dr. A. Macfarlane 

 (Phil. Mag. Dec. 1880), we find that if the dielectric be 

 hydrochloric-acid gas and the gaseous layer be a centimetre 

 thick or more, the electric energy per cubic centimetre will be 

 less than 1000 in C.G.S. units; while the amount of energy 

 necessary to decompose 1 cub. centim. of hydrochloric- acid 

 gas is more than 4 x 10 9 in the same units. Thus only about 

 one four-millionth part of the gas would have to be decom- 

 posed in order to exhaust the energy of the electric field. This 

 quantity, though so small, is yet probably much larger than 

 would in reality be required, as the work which absorbs the 

 energy of the electric field is the splitting- up of the molecules 

 of the hydrochloric-acid gas into hydrogen and chlorine 

 atoms. These atoms will combine with each other to form 

 molecules of hydrogen and chlorine respectively, and will give 

 out heat in so doing, which will heat the gas, but will not 

 restore the electric energy. Now the heat of combination of 

 hydrogen and chlorine when they form hydrochloric acid is 

 not the same as the heat required to split up hydrochloric acid 

 into atoms of hydrogen and chlorine, but is equal to the latter 

 quantity minus the heat given out when the atoms of hydro- 

 gen and chlorine combine to form molecules of hydrogen and 

 chlorine respectively. The determinations by Prof. E. Wiede- 

 mann of the heat given out when hydrogen atoms combine to 

 form molecules, and by Prof. Thorn sen of the same quantity 

 for carbon atoms, seem to show that these quantities are greater 

 than the heat given out in ordinary chemical reactions, and 

 thus that these latter quantities are the differences of quanti- 

 ties each much greater than themselves. Thus to decompose 

 hydrochloric acid into hydrogen and chlorine would require 

 much more energy than the mechanical equivalent of the heat 

 of combination of hydrogen and chlorine. 



