144 SCIENCE PROGRESS 
corresponds to the melting point. At the outset of his dealings 
with double salts, Van’t Hoff had drawn attention to the fact 
that co-existence of the three phases double salt, one simple 
component, and solution, can occur only within a certain range 
of temperature, and that there is only one temperature (the 
transition point) at which there can be equilibrium between 
the solution and the three solid phases double salt and two 
simple salts. 
Soon after, Roozeboom,! applying the principle and the 
terminology of the phase rule, recognised the transition point 
TABLE AYU 
THE QUINTUPLE PoINT 
in the non-variant system composed of three components (e.g. Na,SO,; MgSO,; H,O) 
present in five phases: 
(2) Simple Salt M’A . 2'H,0, e.g. MgSO,. 7H,O 
(3) Double Salt MmM’nAm +n- 9H,O, 
e.g. NaMg(SO,)..4H,0 
(4) Liquid: Saturated Solution of MA and M’A, 
e.g. Na,SO, and MgSO, 
(5) Gaseous: Water Vapour 
iS, 
als {inp Salt MA . £H,O, e.g. Na,SO,. 10oH,0 
wW 
I. to V. are Vapour-pressure Curves for the Five 
Univariant Four-phase Systems. 
I. for (1), (2), (3) and (5) 
II. ,, (1), (2), (4) 5, (5) 
IIT. 5, (2), (3)s (4) 95 (5) 
» (1), (3), (4) 55 (5) 
x3 (1), (2), (3) 5, (4) 
Pressure 
<5 
Temperature Vv 
of a double salt as the quintuple point of a non-variant system 
composed of three components present in five phases, this point 
being the intersection of curves for five univariant systems, each 
composed of four phases (Table III.). 
Roozeboom’s paper was followed in quick succession by the 
publication of much experimental and theoretical work on the 
lines by him initiated? The greatest amount of attention was 
“Die Umwandlungstemperatur bei wasserhaltigen Doppelsalzen und ihre 
Loslichkeit,” Zs. Dhystk. Chem. 2, 1888, p. 513. 
* Meyerhoffer, “Die gesattigten Lésungen der Verbindungen von Cuprichlorid 
mit Kaliumchlorid,” Zs. physik. Chem. 5, 1890, p. 96; “Zur Kenntnis der 
Umwandlungsintervalle,” zézd., 9, 1892, p. 641. Vriens, “ Dampfspannung des 
Kupferkaliumchlorids und seiner Lésungen,” zd¢d., 7, 1891, p. 194. Schreine- 
