167 
the melting-point, and has a yellow brownish colour at the. tem- 
perature of the room, becomes darker with rise of temperature; at 
about 50° it is already dark brown, and the transparency diminishes 
gradually with ascending temperature. The vapour which has a 
lighter colour at equal temperature on account of its slighter density, 
also gets darker with increasing temperature, so that above 100° the 
meniscus between liquid and vapour can hardly be distinguished. 
Hence the critical phenomenon of this darkbrown liquid and vapour 
has not been directly observed. The only value of the critical 
temperature recorded in the literature, has, accordingly, not been 
determined by an optical, but by another way. 
For the determination of the critical temperature NADrJDINE *) made 
use of a very ingenious method, which, however, has not yielded 
accurate results. A tube was provided with a balance-knife in the 
middle so that it could execute regular oscillations round the state 
of equilibrium. If now the tube is filled with nitrogen tetroxide, 
regular oscillations are impossible, the tube inclines to the side where 
the liquid is. With rise of temperature above the critical however, 
the tube fills homogeneously, and gets in equilibrium. The tempera- 
ture, at which this setting in of the equilibrium takes place, was 
considered to be the critical temperature; it amounted to 171,2° C. 
We have, however, succeeded in observing the critical phenomenon 
directly optically. With incident and transmitted light there is nothing 
to be observed of the critical phenomenon in our tubes of about 
3 mm. bore (thickness of the wall 3 mm.). Even the use of an are 
lamp did not bring a change. When, however, we threw the light 
on the tube (in a bath of paraffin oil), and directed our eye so 
that the light that was reflected on the inner wall of the tube, could 
reach our eye, we could clearly distinguish the demarcation between 
liquid and vapour. In one of the tubes we saw the meniscus quickly 
shift to one of the extremities on rise of temperature, and disappear 
suddenly. In another tube, the volume of which pretty well agreed 
with the critical volume of the filling, the line óf demarcation dis- 
appeared suddenly about in the middle of the tube. Both tubes 
yielded 158,2° C. for the temperature at which the demarcation 
between liquid and vapour disappeared. We have repeatedly carried 
out these determinations independently of each other; the obtained 
values agreed within 0,2°. So the critical temperature amounts to 
158,2°, and accordingly differs considerably from the value given by 
NADEJDINE. 
1) Beibl. 9, 721 (1885). 
