142 



SCIENCE PROGRESS 



sulphates of sodium and magnesium are rubbed together and 

 heated to above 22 the mass liquefies as the result of the 

 formation, accompanied by splitting off of water, of the double 

 sulphate astrakanite. Conversely, finely powdered astrakanite, 

 when rubbed with a small amount of water at a temperature 

 below 22 , soon solidifies to a hard mass, which is a mixture of 



TABLE II 



Examples of Double Salt Formation and Decomposition 



/. Astrakanite 



, Na.,S0 4 . ioH„0\_>/Na,Mg(S0 4 )o . 4H,0 

 + MgS0 4 . 7 H 2 6 |«_\ ;i 3 H 2 Q 



//. Cupric Dipotassium Chloride 



CuKCU 

 + KC1 \~T K,CuCl 4 . 2H 2 

 + 2HX> ) <r ~ 



III. Copper Calcium Acetate 

 CuAc 2 .H 2 0~| 

 + CaAc* . HX> \~7_ CuCaAc 4 . SH„0 

 + 6H 2 " J^" 



IV. Seignette (or Rochelle~) Salt 



2Na 2 Ta.2H 2 Cn 

 + 2K,Ta . |H 2 \~7_ 4NaKTa . 4H 2 

 + nH 2 " J^~ 



V. Scacchis Salt 



</.NaNH 4 .Ta.4H.,0\->f(Na. NH 4 . Ta), . 2H.X) 

 + /.NaNH 4 .Ta.4H:,0/<-\. + 6H 2 



Transition Point and 

 Transition Interval. 



D.S. formed at 2r 5° 

 and partially de- 

 composed by water 

 between 21 '5° and 



25°- 



D.S. decomposed 

 at 92 . 



D.S. decomposed 

 at 75°- 



Double tartrate de- 

 composed at 55 

 and partially de- 

 composed by water 

 between 41 and 

 55°- 



Racemate (double) 

 formed at 27 

 from two tartrates 

 (double), not de- 

 composed by 

 water. 



Heat effect 

 in forma- 

 tion of D.S. 



Heat 



absorbed. 



Heat 

 evolved. 



Heat 

 evolved. 



Heat 

 evolved. 



Heat 



absorbed. 



Change of 

 hydration 

 in forma- 

 tion of D.S. 



Water 

 split off. 



Water 

 absorbed. 



Water 

 absorbed. 



Water 

 absorbed. 



Water 

 split off. 



the two constituent sulphates, a change which does not occur if 

 the salt and the water are kept above 22 . He also quotes the case 

 of the green acetate of copper and the white acetate of calcium, 

 which, when powdered finely and moistened with water in the 

 ratio of the equation CuAc 2 . H 2 -I- CaAc 2 . H 2 + 6H 2 0, solidify 

 at ordinary temperature to a hard blue mass of the double salt ; 

 the latter, when heated to above 78 , again decomposes into the 

 two constituent salts, a transformation which makes itself evident 



