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LXXII. On a Class of Double Sulphates, containing Soda 



and a Magnesian Oxide. By A. R. Arrott, Esq.* 

 Vl/THEN a mixed solution of sulphate of soda and any of 

 the magnesian sulphates is allowed to crystallize by 

 spontaneous evaporation, these salts always separate from their 

 solution apart and in their ordinary form, no double salt being 

 produced. 



I find however that if the solution be kept at a temperature 

 exceeding 100° F., the temperature at which anhydrous sul- 

 phate of soda begins to be deposited, a double salt is formed, 

 and this is true of all the magnesian sulphates. The double 

 salts may generally be procured in well-defined crystals, except 

 that of copper, which is usually deposited as a crystalline crust. 



One member of this class of double sulphates, namely the 

 sulphate of magnesia and soda, was obtained by Dr. Murray 

 in the manufacture of sulphate of magnesia from sea water, 

 being produced accidentally during the evaporation of the 

 liquor ; but he seems not to have been aware of the circum- 

 stances of its formation. 



Several soda salts of the same class were also obtained by 

 Mr. Graham by a process which he has described, namely, 

 by mixing strong solutions of bisulphate of soda and the mag- 

 nesian sulphate in atomic proportions; the double salt sepa- 

 rated by crystallization in the course of a few days, at the or- 

 dinary temperature. The reason why no double salt of soda 

 is formed at low temperatures seems to be the affinity of sul- 

 phate of soda for water, and the consequent formation of a 

 hydrated sulphate of that base, which cannot enter into such 

 combinations. 



This interference is, however, prevented by the use of a 

 high temperature, at which, as is well known, sulphate of soda 

 is deposited from its solution in the anhydrous condition, and 

 probably therefore exists dissolved in that state. 



The method which succeeds best is to dissolve the salts to- 

 gether in equivalent quantities, and to evaporate at a tempe- 

 rature of 130°. In this way I have formed the double salts 

 of soda with magnesia, zinc, iron, copper and manganese. 



The quantity of water contained in 100 parts of these salts 

 was — Experiment. Theory. 



Sulphate of magnesia and soda 

 zinc 

 iron 

 copper 

 manganese 



* Communicated by the Chemical Society; having been read January 

 1, 1844. 



