76 
MR. P. V. SEVAN ON THE COMBINATION OF HYDROGEN 
But from the law of mass action we should expect that the rate of combination 
would he proportional to the square of D or to the product of the densities of 
hydrogen and chlorine, so that we should have 
^ 1 ' 0 
And this gives for C for the same periods as before 
•00023, -00038, -00024, -00045. 
These numbers are not constant enough for any definite conclusion as to the action 
in this case following the mass action law ; ljut remembering the great eflect of 
impurities, and noticing that in the bulbs used in these experiments from 4-2 to 6 per 
cent, of air was present, it is surprising that the values of C are so nearly equal. 
Gautiet! and Helier also tiied the effect of increase in the j^roportion of one or 
other of the gases in the mixture, and found that the rate of action was increased, 
thus contradicting a result of Bunsen and Boscoe. The principle of mass action 
would lead us to expect a decrease in the rate of combination from this cause. For 
suppose D, D' the concentrations of hydrogen and chlorine in the mixture, x the 
concentration of hydrochloric acid gas, then 
= ODD', X + D + !>' = F 
dt 
and D' — D == A, a constant, 
and therefore _ c c _ ^ 2 > 
dt 4 ^ ^ ’ A 
which is obviously a maximum when A = 0, or when hydrogen and chlorine are 
j)resent in molecular projDortions. 
We must accept Bunsein and Roscoe’s result in preference to that of Gautier and 
Helier, from the very much better conditions of the experiments of the former 
observers. In Gautier and Helier’s exjieriments difierent bulbs were used for each 
observation, and, further, the amount of impurity present was very indefinite. It is 
very doubtful if experiments made in difierent vessels can ever be more than very 
roughly comparable, as the determination of the factor required to allow for loss ot 
light ]jy reflexion and by alisorption by the glass of the bulb is impossible. The 
absorption of ultra-violet light by the glass makes a large difference in the rate of 
combination. With l)ull)s filled with the mixture and exposed, side by side, to difiiise 
daylight, very difierent proportions are combined after the same exposure. 
Gautier and Helier believe the action to take place in more than one stage, 
oxides of chlorine being formed which are reduced by the action of the hydrogen 
