596 JOHN JOHNSTON AND PAUL NIGGLI 
with B and C. The combination (A B, solution Q) can result 
only when the gross composition is within the triangle ABQ; the 
combination (BC, solution R), only when the composition falls 
within BCR. The triangles APQ, BOR, and CRS are the regions 
in which the two-phase combinations of solutions with A, B, or 
C, respectively, are stable. 
TACHHYDRIT 
'0 
20 
- 
CaCi2 6H:0 
Fic. 2.—Portion of diagram for the ternary system H,O—MgCl.-CaCl, repre- 
senting schematically the solubility isotherms at 23° under a pressure of 1 atm. 
If, starting from the equilibrium ACBL at 21°95, we raise the 
temperature by 0°2, we must raise the pressure by 11.8 atm. 
in order to prevent the disappearance of any of the four phases; 
this relation was experimentally determined, so that the direction 
of this particular 4-phase line at 22° is known. Investigation of 
the combination BCL at higher temperatures (at 1 atm.) showed 
that at 25° a second 4-phase line is met; at this temperature, 
