148 SCIENCE PROGRESS 
II. The methods named zndirect are all based on the principle 
that at the transition point—that is, at the temperature which 
marks equilibrium between various states, when 
mMA + 2M’A + £H;0 2 M,.M,An+2 + HO 
all the physical conditions for the two systems must be the 
TABLE WV 
EXPERIMENTAL DETERMINATION OF THE TRANSITION POINT IN 
THE FORMATION AND SPLITTING UP OF DOUBLE SALTS 
= Results for Astrakanite 
; I. Direct Methods Na,SO, . 10H,0 
Based on finding the temperature at which occur the +MgSO,.7H,0O 
physical changes (in volume, heat of formation, colour, 
state of aggregation) accompanying the transformation ¢Na,Mg(SO,),. ie 
of one system into the other, +13H,O 
mMA+nM’A+/H,0 2 M,.M':Amin+LH2O 
Gi)/Milatometnc *. 2 1) S. ees 2OOtEO aie 
(ii) Thermometric : ; : ; , 22°5 to 22°6 
(iii) Observation of change in colour or state of 
aggregation : ’ 5 é ; . between 20 and 25 
LT. Indirect Methods 
Based on finding the temperature at which the equi- 
librium conditions are the same for the systems: 
Double Salt + Single Salt MA + Saturated Solution, 
Double Salt + Single Salt M’A + Saturated Solution, 
Single Salt MA+Single Salt M’A+ Saturated Solution, 
z.e. when all three or any two of these saturated solutions 
have identical properties (e.g. composition, vapour pres- 
sure, etc.) : 
(i) Solubility : : ; : : : 22 
(ii) Tensimetric . : : : : ; . 20°4 
(iii) Electrical : ; : : : : . 20°7 to 20°9 
(iv) Optical. 
same ; such conditions are the composition, the vapour pressure, 
the electrical conductivity and electromotive force, and the | 
refractive index of the saturated solutions of the following three 
combinations of two solid phases: (i) the two single salts, 
(ii) the double salt and the single salt MA, (iii) the double salt 
and the single salt M’A. Curves are constructed which represent 
for two or for all “ree of these combinations the variation of 
these properties with variation in temperature (Tables III. 
