Combination of Hydrogen and Chlorine. 119 



therefore determined by the spark, just as combination 

 between dry hydrogen and oxygen is determined by the 

 spark. 



On the other hand, the action of light on a mixture of 

 hydrogen and chlorine is affected by the presence of 

 moisture. In the course of our experiments on hydrogen 

 and chlorine (which were begun in 1882) we have on several 

 occasions noticed the difficulty of exploding dry hydrogen 

 and chlorine by a flash of light. The same observation has 

 been made by other chemists, of whom we may mention 

 Mr. W. A. Shenstone. But the fact was not, so far as we 

 know, published until 1887, when Pringsheim,* in his 

 admirable paper on the chemical action of light on hydrogen 

 and chlorine, stated that the mixture when long dried by 

 phosphoric acid exploded only after a few seconds in sun- 

 light and with a weak clicking sound ; and that in diffused 

 daylight the formation of hydrogen chloride was greatly 

 retarded. 



In order to compare the action of light on the wet and 

 dry mixture of hydrogen and chlorine, we filled a row of 

 six bulbs with the electrolytic gas, and sealed them up. In 

 the last two bulbs plugs of anhydrous phosphoric acid were 

 placed, so that the gases were sealed up in contact with the 

 drying agent. After lying in the dark some weeks, a wet 

 and a dry bulb, screened one from the other, were exposed 

 to the light of a magnesium " flash " placed six feet from the 

 two. Neither exploded ; but on firing a similar flash five 

 feet from the two, the wet bulb exploded loudly, the dry 

 bulb was unaffected. On firing flashes at 4 and 3 feet from 

 the dry bulb, no action was observed. Fired at 2 feet from 

 the dry bulb, the flash produced a loud explosion. In other 

 experiments similar results were obtained. When the 

 electrolytic gas was well dried before admission to the bulb, 



*Annalen iUr Pliysik, 1887. N.F., xxxii.. p. 421, 



