(373) 
anhydrous salts. Among these many are known whose curve of solu- 
bility may be continued up to their melting point C. If this melting 
point is situated far enough above 100°, the solubility at this tem- 
perature is still small enough that the saturated solutions belong 
to the part AB of the vapour pressure curve. If we now draw a 
line VEG parallel to the temperature axis at a height of p = 1 atm. 
this will intersect the vapour pressure curve first in a point Z which 
is situated above 100° and indicates the temperature at which the 
saturated solution has a vapour pressure of 1 atm. 
If, therefore, we start with a saturated solution at a lower tem- 
perature, the vapour pressure will rise from A to Z owing to the 
continuous solution of solid salt caused by the heating, but in an 
open vessel if will not be able to get above that point, for, on 
further heating, the solution will evaporate to dryness while the 
temperature remains constant. For we have got here three phases, 
solid salt, solution and vapour at a constant pressure of 1 atm. 
The same result is obtained in a still more striking manner when 
we start with a dilute solution boiling at #. Owing to the boiling 
its concentration will increase and so we travel along all the points 
of the line DE until the saturation point / is reached, The boiling 
point does not rise any further because in proportion as the water 
evaporates solid salt is deposited and the concentration remains 
unchanged. 
This state of affairs has been repeatedly observed by different 
investigators. We were satisfied with convincing ourselves that the 
temperature in ZZ remains very constant if the regularity of the 
boiling is promoted by passing steam, and the source of heat is an 
oilbath and not a naked flame. We will call the point £ the first 
boiling point. 
As soon as the solution is quite evaporated, the temperature may 
rise and so we further proceed along the line ZF while the salt 
remains exposed to water vapour of 1 atm. This line ZF now 
intersects the vapour pressure curve of the saturated solution for 
the second time at #. As soon as this point was exceeded we should 
arrive, along FG, in the region of the unsaturated solutions. Conse- 
quently a new solution must form in F£, 
In F, therefore, a solution is formed from solid salt and aqueous 
vapour of 1 atm. pressure, which being just the reverse of the 
evaporating of the saturated solution on branch BC, will absorb 
heat. Consequently, on supplying heat the temperature in / remains 
again constant until all the salt has been solved for which generally 
but very little water is required. 
25 
Proceedings Royal Acad, Amsterdam, Vol. IV, 
