as connected with the Theory of Substitutions. 359 
ture p. The two ground glass plates were now moved on one 
another in such a manner that they mutually closed one another. 
The vessel A was therefore filled with dry chlorine, and the ves- 
sel Be with an equal volume of dry hydrogen, without commu- 
nicating 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 manner. 
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 sliding 
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 pa- 
per 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 Be of each set were filled with dry hydrogen in the man- 
ner described. In each instance, as soonas the plates were mov- 
ed on each other so as to confine the hydrogen, and they 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 a saucer P, (Fig. 8.) The two sets of tubes being 
held steadily in a proper position by the aid of a wooden frame 
QR. The two sets of tubes now differed from one another in 
nothing 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 apertures 
coincided, as shown in Fig. 9. The hydrogen now rose through 
the hole into the upper vessel, the chlorine descending 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 tem- 
perature 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 tubescof 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. 
