AND CHLORINE UNDER THE INFLUENCE OF LIGHT. 
93 
There are three causes to which this final contraction may be attributed :— (I) The 
cooling of the gas which has lieen lieated by the comhination ; (2) Continuation of the 
action, hydrochloric acid being formed for some time after darkening; (3) Absorption 
of hydrochloric acid formed while the light was acting. 
Bunsen and Roscoe concluded that the effect was due to the first cause, but the 
amount of contraction in their experiments was too small for much reliance to be 
placed in their result. 
With the platinum thermometer used in earlier experiments it was easy to decide 
how much of the contraction was due to the first cause. The gas was illuminated; 
on darkening, the temperature was found to fall as the contraction proceeded, and it 
was found that the contraction agreed, within the limits of accuracy of the 
experiment, with that corresponding to the fall in temperature. 
The following two tables give the results for two series of experiments of this kind. 
In the first column is the observed fall in temperature, in the second the change of 
volume due to this fall, in the third the observed change of volume, and in the fourth 
the ratio of the numbers in the third and second columns. 
Change in 
temperature in ° C. 
Change of volume 
corresponding 
to change 
of temperatime in 
first column. 
Observed change 
of volume. 
Ratio. 
•28 
230 
240 
1-04 
•41 
336 
320 
•95 
•41 
336 
320 
•95 
•42 
344 
360 
1 -05 
•42 
344 
395 
1-15 
•35 
287 
315 
1-12 
•43 
353 
353 
1-00 
•44 
361 
335 
•93 
The mean of the numbers in the last column is f ’024. 
