90 
MR. P. V. BEVAX ON THE COMBINATION OF HYDROGEN 
temperature is fully accounted for by the heat of formation of the hydrochloric acid. 
We have already seen that the expansion is larger compared with the contraction, 
representing hydrochloric acid formed, the smaller the quantity of acid formed and 
tlie shorter the time of insolation; but the ratio of these two quantities always falls 
considerably below 10 h. For the bulb used, a rise of temperature of -01° C. 
corresponds to an expansion of about 9 milhms. of the scale, and in no case was the 
expansion observed as large as that value which would correspond to the observed 
rise in temperature. The reason that this value of the expansion is not observed is 
that the motion of the index is relatively slow, and the gas cools so quickly, with 
very short exposures, that the index cannot register the expansion corresponding to 
the total rise in temperature. 
The conclusions then with regard to the initial expansion period are :— 
(1) The expansion is due to rise in temperature of the gas mixture ; 
(2) This rise in temperature is caused by the heat liberated in the formation of 
hydrochloric acid. 
The initial expansion period is therefore nothing more than a part of the period of 
induction as studied by Bunsen and PbOSCOE; and the condition necessary for the 
existence of the Draper effect is that the intensity of the light should be such that 
the combination can begin rapidly enough for the heat evolved to produce an 
observable effect. 
§ 5. The Expansion of Chlorine alone under the Influence of Liflit. 
In connection with this subject the results of experiments of Budde* * * § and Baker,! 
on the expansion produced in chlorine alone by the influence of light, are of interest. 
Budde found that under the influence of light damp chlorine expanded. In his first 
paper! he states tliat the expansion cannot be due to a rise of temperature, but in a 
later series of experiments he found that a rise in temperature did occur, and although 
the rise in temperature did not agree with the expansion very well, be concluded 
that the expansion was a direct result of the heat resulting from the absorption of 
light. Baker found that perfectly pure and dry chlorine did not change its volume 
when exposed to light. 
Pringsheim§ showed that this expansion was very different from that in the 
Draper effect. With the most intense sparks procurable, there is no observable 
expansion in chlorine alone. 
It seemed worth while, with the apparatus already described, to settle the question 
* Budde, ‘Phil. Mag.,’ 1871, vol. 42, p. 290. 
t Baker, ‘Brit. Ass. Rep.,’ 1894. 
\ Budde, ‘Pogg. Ann.,’ vol. 6, 1873, p. 477. 
§ Pringsheim, ‘Wied. Ann.,’ 1887, vol. 32, p. 413. 
