AND CHLORINE UNDER THE INFLUENCE OF LIGHT. 
80 
The curve represented in fig. 7 has for abscissae the final contraction, indicating the 
amount of hydrochloric acid formed, measured in millimetres of the capillary tube, 
and for ordinates the rise in temperature observed. The j)oints all lie about a line 
which at first is straight; and the deviations from this line are not greater than are 
to be accounted for by the unavoidable errors of exjDeriment. 
In this experiment then, there is an observed rise of temjoerature which for the 
cases when the amount of hydrochloric acid formed is small is proportional to the 
amount of hydrochloiic acid formed. The time of illumination for these exjieriments 
was very short for the observations represented by the straight part of the curve, the 
illumination only lasting for a fraction of a second. (The longest illumination was 
about 1‘5 second.) 
With the galvanometer made as sensitive as possible, a rise of temperature was 
always indicated with the shortest illuminations obtainable liy means of an electric 
spark. Even when the illumination was adjusted so that the only observable effect 
on the index was an expansion of a small fraction of a millimetre, a rise of temperature 
could always be detected. • 
§ 4. Conclnsions. 
We conclude that under all conditions there is associated with the initial expansion 
a formation of hydrochloric acid and also a rise in temj^erature. 
Taking Thomsen s value 22,000 cal. for the heat of formation of hydrochloric acid, 
the increase of volume of an equal mixture of hydrogen and chlorine, supposing all 
the heat expended in expanding the resulting hydrochloric acid, is such that the new 
volume equals 10'5 times the old volume. In the vessel used, in the experiments just 
detailed, a formation of liydrochloric acid, coiTespondiug to a contraction of 1 millim. 
of the index, would cause a rise in temperature in the gas of very nearly -01° Ct In 
all cases the actual rise observed was less than this, so that the observed rise in 
VOL. ecu. — A. XT 
