94 
MR. P. V. BEVAN ON THE COMBINATION OF HYDROGEN 
j Change in 
temperature in ° C. 
Cliange of volume 
corresponding 
to change 
of temperature in 
finst column. 
■ 
Oliserved change 
of volume. 
Ratio. 
, 
1 
•08.5 
69 
53 
1 
•77 ! 
j -101 
83 
98 
1-18 : 
•146 
120 
150 
1-25 
1 -192 
157 
160 
1-02 
1 -283 
232 
232 
1-00 
j -308 
253 
250 
•99 
•321 
263 
316 
1-20 
•341 
280 
280 
1-00 
•401 
329 
360 
D09 
•401 
329 
340 
1-03 
•462 
380 
398 
1-04 
•481 
394 
400 
1-01 
•504 
413 
402 
•97 
•640 
525 
455 
•87 
The mean of the numl^ers in the last column is 1 • 030. 
The contraction therefore, after the light is cut off, is nearly all clue to cooling. If 
any hydrochloric acid is absorbed after darkening, it is very little, and we conclude 
that the formation of hydrochloric acid stops almost instantaneously when the light 
is cut olf, and that the hydrochloric acid is absorbed almost as soon as it is formed. 
3. The Period of Induction. 
The problem is now simplified to a great extent. The Draper effect and the final 
contraction have been shown to be due to temperature effects, which complicate the 
initial and final phenomena. Fig. 4 represents the general course of the induction 
which takes place under all conditions of illumination. On illumination, hydrochloric 
acid is formed at first very slowly, the rate of formation increases, tlie acceleration of 
the rate of formation increases until a maximum acceleration is reached; then the 
acceleration decreases to zero, when the rate of action becomes constant, if the gases 
are present in the mixture in molecular proportions; if there be excess of one gas, the 
final rate gradually diminishes as the relative quantities of the two gases become 
more and more different. 
We shall at present confine ourselves to the case when the gases are present in 
approximately molecular proportions, so that all through the experiments the 
amounts of uncombined gases are present in the same relative quantities. 
§ 1. Prolongcttion of the Period of Induction. 
In Bunsen and Roscoe’s experiment the period of inertness was never found to be 
more than 6 or 7 minutes. If, however, a much less intense light be used the 
