997 
in. At 552°, bowever, the pressure was soon constant, so that it 
could be read accurately. 
After this reading the apparatus was cooled to examine how great 
the pressure of the remaining gas was at the ordinary temperature. 
We were, namely, convinced that now too gases had escaped, and 
wanted to apply a correction for this. At the ordinary temperature 
a pressure of 30 em. Hg was, indeed, still found, which corresponds 
to a pressure of 1,1 atm. at 552°. If we correct the vapour pressure 
at 552° with this pressure, the vapour tension of black phosphorus 
at 552° becomes 20.7 atm., the vapour tension of violet phosphorus 
amounting to 21.0 atm. 
In the experiments discussed here we have, therefore, tried to 
expel the volatile kerosene residues from the black phosphorus by 
evaporating the substance in vacuum at 260° resp. 300°. In this 
way of purification we could of course, hardly go to higher tem- 
peratures than 300°, because else a too great loss through evaporation 
would take place. 
As it was, perhaps, necessary for a most thorough removal of the 
kerosene to heat to higher temperatures, we made at last the follow- 
ing experiment. 
Black phosphorus was placed in a thick-walled tube of sparingly 
fusible glass; then the tube was exhausted and soldered to. After 
this the tube was suspended in a bath of molten KNO,—NaNO,, 
which bath was heated up to 550°. 
After this heating had been continued for half an hour, the tube 
with phosphorus was taken out of the bath, in which white phos- 
phorus deposited out of the vapour. After cooling the tube was 
opened under CS,, and the mass extracted as usual with CS, and 
ammonia, etc. 
This preparation, of which we expected a favourable result, yielded 
what follows for the determination of the vapour tension. 
Vapour tension in atmospheres 
‘hem peratirel | man Sen In SS 
Black P Violet P 
i152 8.5 (increasing) 10.5 
553 19.3 (increasing), 21.5 
567 28.6 (constant) | 28.5 
578 39.5 35 
At the temperatures 515° and 553° the internal equilibrium had 
