116 
MR. P. V. SEVAN ON THE COMBINATION OF HYDROGEN 
When the combination takes place in a vessel under constant pressure and where 
the hydrochloric acid is absorbed as soon as it is made, the concentration of the 
H2O, H2, and CI3 molecules can be considered to be constant, as the intermediate 
compounds are formed only in small quantities. If, however, as in Gautier and 
Helier’s experiments, the action takes place in a closed vessel, the active masses of 
the hydrogen and chlorine diminish, and this must be taken into consideration. We 
shall assume that the water-vapour has constant concentration throughout, as it is 
liberated in the final stage of the action.* 
Taking this second case first, let ^ be the number of chlorine or hydrogen molecules 
per unit volume (supposing the gases present in equivalent proportions). Let x be 
the number of molecules of the formula HgO CI3, y the number of molecules 
H3O, CI2, Hg, and the number of molecules of hydrochloric acid. Then, from the 
law of mass action for the formation of the HCl molecules. 
dzjdt = Cjy, 
Cj being a constant dependent on the stability of the H3O, Clg, H3 molecules; 
dy/dt — — dzjdt, 
for the rate of formation of the HgO, CI3, H2 molecules is proportional to the 
concentration of the H2O, CI2 molecules and that of the II2 molecules. And also for 
every two molecules of HCl formed, one molecule of H2O, CI2, H3 is lost (we assume 
that the number of H3 molecules is not appreciably different from the number of CI2 
molecules, ?.e., that x can be neglected when compared with ^). 
And finally we have in a similar manner 
dxjdt = Cgf — c^x^. 
And, since 2 z is the number of HCl molecules formed, 2^ the total number of H2 
and Cln molecules, and there is no change in the total number of molecules (the 
number of intermediate molecules being negligible compared with those of hydrogen, 
chlorine, and hydrochloric acid), 
^ z ■= constant = n. 
For the case when the hydrochloric acid is absorbed as soon as it is produced, the 
concentration of the chlorine and hydrogen molecules remains constant and the 
equations are of the form. 
dzjdt — c^y 
dyjdt — c^x — c^y 
> 
dxjdt = Cg — c^x 
( 1 ). 
* This assumption is perhaps not justifiable for some of Gautier and Helier’s experiments, as the 
gases were partially dried, and so there was only a small quantity of water-vapour present. 
