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: il 
EXAMPLES OF DouBLE SALT FORMATION AND DECOMPOSITION 
Change of 
Transition Point and ee hydration 
Transition Interval. |,- in forma- 
tion of D.S. tion of D.S. 
I. Astrakanite 
, Na,SO,. 10H,0|—> f Na,Mg(SO,), . 4H,O D.S. formed at 21°5°| Heat Water 
MgsSO,. 7H,O fe +13H,O and partially de-| absorbed. | split off. 
composed by water 
between 21°5° and 
ZA 
II, Cupric Dipotassium Chloride 
CuKCl, D.S. decomposed Heat Water 
+ KCl 2 K,CuCl, . 2H,O at 92°. evolved. | absorbed. 
+2H,O 
ITI. Copper Calcium Acetate 
CuAc,. H,0 | = D.S. decomposed Heat Water 
+CaAc, . H,0 7 27 CuCaAc,.8H,O at 75°. evolved. | absorbed. 
+6H,O J 
LV. Seignette (or Rochelle) Salt 
2Na,Ta.2H,O =a Double tartrate de-| Heat Water 
+2K,Ta.3H,O ee 4NaKTa. 4H,O composed at 55°| evolved. | absorbed. 
+11H,O and partially de- 
composed by water 
between 41° and 
55° 
V. Scacch?s Salt 
d.NaNH,.Ta.4H,O\->/{(Na. NH,.Ta),.2H,O | Racemate (double) Heat Water 
+/.NaNH,.Ta.4H,Of<-\ +6H,O formed at 27° | absorbed. | split off. 
from two tartrates 
(double), not de- 
composed by 
water. 
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.. H,O + CaAc,. H,O + 6H.O, 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 
