999 
theory of allotropy. but it is never observed and therefore we think 
it more prudent to suppose, for the present at least, that the inter- 
section does not exist. We will suppose here, that the vapour tension 
line of the black phosphorus thus as a whole lies above that of the 
violet phosphorus. 
The investigations are continued and the results will be communi- 
cated in a second paper on this subject. 
Accordingly the general result is this that below 550° a very slow 
increase of pressure was observed, which is unquestionably partly a 
consequence of an internal transformation, just as this was observed 
for the disturbed violet phosphorus. A little above 550° the pressure 
generally became pretty soon constant, and it was exactly this con- 
stant pressure that pretty well agreed with the pressure of the violet 
phosphorus. Above 570° the vapour tension of the black phosphorus 
rose considerably above that of the violet phosphorus. 
4. Determination of the triple point temperature of the black 
phosphorus. 
To determine the temperature of the melting-point under the vapour 
pressure, we have made the following experiment. Three tubes ot 
thick-walled sparingly soluble glass were filled with three different 
substances, tube a was filled for */, with the purest preparation of 
black phosphorus; tube 4 with the same black phosphorus + 1 °/, 
iodine, and tube ec with violet phosphorus + 1 °/, iodine. 
These three tubes were fastened together by means of copper 
wire, and suspended in such a way that they could be given an 
oscillating motion. To make this suspension apparatus heavier, a 
copper bar of the same length as the glass tubes was fastened at 
the bottom. 
Then this apparatus was now placed in a bath of a molten mixture 
of KNO, + NaNO, of + 200°, in which a stirrer was placed, which 
brought about a uniform temperature. 
After a gauged thermo-element had been placed 
in the bath, which element had been inserted 
into one of the branches of a Wheatstone bridge 
in the usual way, the nitrate bath was slowly 
raised to higher temperature. 
In preliminary experiments it had appeared 
that when black phosphorus is rapidly heated, 
this substance does not begin to melt until a temperature is reached 
5° or 10° above the triple point temperature of the violet phosphorus. 
As it was now supposed that under these circumstances the velo- 
