73 
AND CHLORINE UNDER THE INFLUENCE OF LIGHT. 
action of the light, while in Bunsen and Roscoe’s experiments the gases were 
bubbled through water into the “ insolation ” vessel after the preliminary exposure to 
light. (See later, p. 101). 
Fremy and Becquerel^^^ confirmed Draper’s result, finding that the effect of light 
on a damp mixture of hydrogen and chlorine is much greater if the chlorine has been 
previously illuminated than otherwise. 
Draper! further discovered that if an intense light, such as that of a spark from 
a Leyden jar, be flashed on the mixture, a rapid expansion takes place, followed by a 
return to, or near to, the original volume. This expansion was also noticed and 
studied by Pringsheim.| Curiously enough, no mention of this effect is made by 
Bunsen and Roscoe. 
Bunsen and Roscoe’s classical work on this action, as a means of measuring the 
chemical effect of light, appeared in 1855 and after. They investigated the 
phenomena from the beginning of illumination to the stage when the velocity of the 
action is constant. This interval of time they called the “ Period of Induction.” At 
the beginning of this period they observed the “period of inertness” noticed by 
Cruickshank and Draper, after this inert period a period of acceleration, and 
finally a time when the velocity reached a constant maximum. On stopping the 
illumination they noticed also a further set of phenomena : a period of retardation 
followed by the complete cessation of the action. Bunsen and Roscoe introduced 
the words “ insolation” and “ insolated,” the gas mixture being insolated when it has 
been exposed to light, and the gas being in a state of insolation as long as the effect ot 
illumination persists. 
The most important work after that ot Bunsen and Roscoe is that of Pringsheim.§ 
The subject of Pringsheim’s investigation was the expansion noted by Draper at 
the moment of illumination with a bright light. He considered the effect of the 
heat liberated in the formation of hydrochloric acid molecules. From the known 
heat of formation of hydrochloric acid we should expect that if hydrochloric acid is 
formed immediately on the incidence of light, an expansion of 10 *5 times the volume 
of hydrochloric acid formed would occur. The expansion, therefore, should be 
proportional to the hydrochloric acid formed. Pringsheim concluded that this was 
not the case. With an instantaneous illumination, obtained by means of a spark, he 
observed a very small expansion followed by a return to the original volume; but no 
decrease of volume afterwards, showing the formation and absorption of hydrochloric 
acid. He concluded, therefore, that the heat liberated by the action is not the cause 
of the expansion. 
Pringsheim supposed that the first action of the light is to cause a dissociation oi 
* Fremy and Becouerel, ‘Wurtz., Diet, de Chimie.,’ 1879, IL, p. 255. 
t ‘Phil. Mag.,’vol. 23, 1843, p. 415. 
t Pringsheim, ‘ Wied. Ann.,’ 1887, vol. 32, p. 384. 
§ ‘ Wied. Ann.,’ 1887, vol. 32. 
