705 
cerfainty that the change of colour from yellow to orange is perfectly 
continuous, and is not attended with a change in erystalline form, 
so that the assumption of a continuous transition from rhombic into 
tetragonal between 127° and 255,5° must be abandoned. It follows 
therefore from Dr. v. p. VrEN's investigation that: (see p. 703) 
1. between 127° and 255°.5 the internal equilibrium very per- 
ceptibly shifts to the side of the p-component, but the colour proves 
already that this shifting is not so considerable that the composition 
of the red modification is reached. 
2. this shifting of the internal equilibrium, which is accompanied 
with a change of colour, is not attended with a change of crystalline 
form. 
These new data rendered it necessary to modify the designed 
figures of the pseudo- and of the unary system somewhat. Led in 
this way to reconsider the problem mercury-iodide, I found that 
the solution was after all exceedingly simple. 
One of the particularities presented by the system Hgl,, is this 
that as has been stated, the concentration of the solid substance at 
the triple point solid-liquid-vapour lies between that of the vapour, 
and the liquid is richest in that component that in the solid red 
modification is represented in the greater quantity. This is a situation 
to which we are not accustomed, though it undoubtedly occurs now 
and then, and it is to be attributed to this that another solution was not 
immediately thought of, which possesses all the advantages of the 
former solution, while the objections advanced against the former 
solution are entirely removed here. 
If we begin with the simplest representation, we take the 7r- 
section of the spacial figures corresponding to a constant pressure, 
and that such a pressure that equilibria with vapour cannot occur. 
Now there are two difficulties. The first that I will discuss here is 
this that the Ze-section bas the shape of the figure indicated in 
thin lines in fig.1. The pseudo components «Hgl, and PHgl,, there- 
fore, give mixed crystals, but the mixing is limited, hence a con- 
tinuous transition between mixed crystals of a different crystalline 
system takes nowhere place. The situation of the unary system in 
this section has been indicated by thick lines, so that the connection 
between the pseudo binary and the unary system is at once clear. 
The internal equilibrium in the tetragonal red modification is 
indicated by the line 5,’ 5, below the transition point. The internal 
equilibrium in rhombic moditication, which continuously passes from 
yellow into orange between 127° and 255°.5, is indicated by the 
line S,S. Comparatively near below the melting-point this line bends 
45 
Proceedings Royal Acad. Amsterdam, Vol. XIX. 
