( 821 ) 
retrograde condensation. So long, namely, as Pa, continues to lie 
higher than 7% the three-phase-line Q,Q,Q',Q,' can lie entirely on 
the upper sheet of the surface of saturation, and can then give only 
rise to the more rapid ascent of the upper branches of the sections 
at given value of z. Above the temperatures at which this more 
rapid rise sets in, the section has then its usual shape, merging 
continuously into the vapour branch, there being only one point 
where the tangent is normal to the 7'‚z-plane. The temperature at 
which the upper branch suddenly begins to rise more rapidly is 
given for every section by the value of 7’ for the points of the 
dotted curve, which, however, must then be broken off before the 
point with minimum value of w. Sections on the right side of Q,' and 
the left side of Q,' present the wholly unmodified shape. Sections 
between Q,' and Q, get the break in the upper sheet at ever lower 
temperature as the point Q,' is approached. In each of these sections 
part of the line of intersection with the three-phase-sheet, i. e. a part 
that belongs to lower temperatures lies above the upper branch. 
The higher part of this line of intersection lies and terminates in the 
heterogeneous region. For the section of Q, this line of intersection 
lies over its full length in this region, and the initial direction at 
Q, then touches the upper branch of the section of Q,, as it proceeds 
unmodified at lower temperatures. For sections between Q, and Q, 
the point of intersection of the upper sheet with the three-phase-line 
lies at ever higher temperatures as we approach Q,. The lower part 
of the tbree-phase-line then lies below this upper branch, and the 
upper part would project above it; but the abrupt change of direction 
taking place at the intersection is so great that even the upper part 
of the three-phase-line remains in the heterogeneous region. So if 
we want to keep a section between Q, and Q, in homogeneous 
liquid state, the pressure must always be greater than the three- 
phase-pressure with the exception of the liquid represented by the 
point of intersection. For the section of Q, the three-phase-line lies 
entirely in the unmodified heterogeneous region, and in the highest 
point it touches the upper branch. For the sections on the left of 
Q, the upper part of the three-phase-line lies again above the upper 
branch, and as such it has lost its significance. Then these sections, 
too, have the sudden change of direction, at which they begin to 
ascend more rapidly, but at ever lower temperature the more we 
go to the left. For the section of the point Q', the whole of the 
three-phase-line lies then above the upper sheet. 
So when the three-phase-line over its full width intersects only 
the upper sheet, there is no further complication ; and this may be 
