MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 297 



quarter of an hour: the chlorine in it becomes modified. 

 Keep the other tube during this time carefully in a dark 

 place ; and now, by lamplight, add to both equal volumes 

 of hydrogen gas. These processes are best carried on 

 in a small porcelain or earthenware trough, filled with a 

 saturated solution of common salt, which dissolves chlo- 

 rine slowly, and to avoid explosions operate on limited 

 quantities of the gases. Tubes that are eight inches 

 long and half an inch in diameter will answer very well. 

 The two tubes now contain the same gaseous mixture, 

 and only differ in the circumstance that one is modified 

 and the other not. Place them, therefore, side by side 

 before a window, through which the entrance of daylight 

 can be regulated by opening the shutter; and now, if 

 this part of the process is conducted properly, it will be 

 seen that the modified chlorine commences to unite with 

 the hydrogen, and the salt water rises in that tube. But 

 the unmodified chlorine shows no disposition to unite 

 with its hydrogen, and the liquid in its tube remains 

 motionless for a long time. Finally, as it becomes slow- 

 ly modified by the action of daylight impinging on it, 

 union takes place. From this, therefore, we perceive 

 that chlorine which has been exposed to the sun will 

 unite promptly and energetically with hydrogen ; but 

 chlorine which has been made and kept in the dark 

 shows no such property." 



This form of experiment may be supposed imperfect, 

 since the chlorine is in a moist condition and confined 

 by water. I have therefore made the following vari- 

 ation : 



I took a tube, A, Fig. 47, six inches long and half an 

 inch in diameter, closed at one end and open at the 

 other, and cemented its open end on a piece of flat plate- 

 glass, M N, one inch wide and two long, ground on both 

 sides, and having a hole,j9, one sixth of an inch in diam- 



