338 Prof. Draper on the Allotropism of CJilmine, 



receive tightly the tube c. In a short time, therefore, B be- 

 came full of dry hydrogen, the surplus escaping through the 

 open aperture p. The two ground glass plates were now 

 moved on one another in such a manner that they mutually 

 closed each other. The vessel A was therefore filled with 

 dry chlorine, and the vessel B c with an equal volume of dry 

 hydrogen, without communicating for the present with one 

 another. 



I had provided two sets of these tubes as nearly alike as 

 they could be made, and operated with them in the following 

 way: — 



In a dark room I filled the tube A of each of them with dry 

 chlorine, in the manner just described, and confined it by sli- 

 ding the plates. One of the tubes was retained in the dark 

 room and kept carefully screened from the light, but the other 

 was set for half an hour in the sunbeams. The chlorine which 

 was in it underwent the specific change which it is the object 

 of this paper to describe. 



After restoring this tube to the dark room, and waiting a 

 few minutes for it to gain the same temperature as the other, 

 the tubes B c of each set were filled with dry hydrogen in the 

 manner described. 



In each instance, as soon as the plates were moved on each 

 other so as to confine the hydrogen, and were released from 

 the cork h of the drying tube K, fig. 7, their lower extremity 

 was dipped beneath the surface of some water contained in the 

 saucer P, fig. 8 ; the two sets of tubes being held steadily in a 

 proper position by the aid of a wooden frame Q R, Q R. 



The two sets of tubes now differed from one another in no- 

 thing but the circumstance that the chlorine of one had been 

 exposed to the sun and that of the other had not. 



The gases were now brought in contact. This was easily 

 done by sliding each pair of ground glasses until their aper- 

 tures coincided, as shown in fig. 9. The hydrogen now rose 

 through the hole into the upper vessel, the chlorine descend- 

 ing through it, mutual and perfect diffusion of the two gases 

 rapidly taking place. This was done by lamplight in the 

 dark room. And now it could be ascertained that the gases 

 were at the same temperature in the different tubes, and that 

 the experiment had thus far been carried on successfully, by 

 the water retaining its level at the same point in the tube c of 

 both sets. If that which had been in the sunshine was 

 warmer than the other, as soon as the apertures coincided a 

 bubble of gas would have escaped through the water, or at all 

 events the level would have changed. 



It remained now to open the shutter of the dark room, the 



