274 MODIFIED CHLORINE. [MEMOIR XIX 



This, therefore, shows that the change which the sun- 

 beams impress upon chlorine .is to a certain extent a 

 permanent change, and, unlike a calorific effect, it does 

 not spontaneously and rapidly pass away. 



III. There are Two Stages in the Phenomenon. 



Let us now make inquiry into the nature of the 

 change thus impressed on the chlorine. This, I shall 

 show, rests in the circumstance of the absorption of rays 

 which correspond in refrangibility to the indigo, and ap- 

 pear to become latent 



In a tube, over salt water, mix together equal volumes 

 of unmodified chlorine and hydrogen gas. Expose it to 

 the daylight, marking the time at which the exposure 

 commences. Watch the level of the liquid in the tube 

 narrowly, and, though stationary for a considerable time, 

 after a certain period has elapsed it will be seen on a 

 sudden to start and commence rising. Observe now 

 hoAV fiir it will rise during a period equal to the time 

 that elapsed between the first exposure and the begin- 

 ning of the rise, and it will be seen that one fourth or 

 half the gases will disappear. 



It is obvious that from the first moment of exposure 

 the rays must have been exerting their influences on the 

 mixture. As will presently be proved, absorption has 

 been all along taking place. There are, therefore, two 

 distinct phenomena exhibited by this experiment. There 

 is a period during which, though large quantities of the 

 dark rays are disappearing, no visible change is pro- 

 duced; there is a second period, during which absorp- 

 tion is accompanied by a remarkable chemical effect 

 the production of hydrochloric acid. From these things 

 we gather that a definite amount of the chemical rays 

 must disappear and become latent before hydrochloric 

 acid can form. The phenomenon is not unlike that of the 



