i 4 8 SCIENCE PROGRESS 



II. The methods named indirect are all based on the principle 

 that at the transition point — that is, at the temperature which 

 marks equilibrium between various states, when 



;«MA + »M'A ±^H,0 j> M w M' ;: A,« + « ±>H»0 



all the physical conditions for the two systems must be the 



TABLE V 



Experimental Determination of the Transition Point in 

 the Formation and Splitting up of Double Salts 



I Direct Methods Kesults/or Astrakanite 



i. uirect mernoas Na 2 S0 4 . ioH 2 



Based on finding the temperature at which occur the \ +MgSO . 7H 2 

 physical changes (in volume, heat of formation, colour, 1^ 



state of aggregation) accompanying the transformation r Na 2 Mg(S0 4 ) 2 . 4H 2 0) 



of one system into the other. \ +i3H,0 / 



«MA + »M'A+/H s O ^ M m M' n A m + n+pH 2 



(i) Dilatometric 2o'6 to 21*6 



(ii) Thermometric 22-5 to 22'6 



(iii) Observation of change in colour or state of 



aggregation between 20 and 25 



II. 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 MA + Saturated Solution, 

 Single Salt MA + Single Salt MA + Saturated Solution, 

 i.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 207 to 20'o. 



(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 three of these combinations the variation of 

 these properties with variation in temperature (Tables III. 



