AMMONIA WITH CARBONIC ACID. 117 



soluble, never more than approximates. When I, in the manner 

 ah-eady mentioned, poured a little water upon the sesquicarbonate, 

 I could not manage to obtain pure carbonate without a small 

 mixture of dissolved bicarbonate ; for, as I precipitated the solu- 

 tion with a solution of the chloride of barium, the filtrated liquid 

 was rendered opalescent by ammonia. 



The affinity between the two constituents in a double salt 

 varies. The carbonate of ammonia is combined so feebly with 

 the bicarbonate in the sesquicai*bonate of ammonia, that water 

 alone may cause a separation of both constituents. We find 

 something similar in several double salts which are composed 

 of a salt difficult, and of one easy of solution. Of the Bro- 

 gniarti (Glauberit), a crystalline double salt of sulphate of 

 lime and of sulphate of soda, the latter dissolves in water and 

 leaves the sulphate of hme undissolved. In the same manner, 

 according to Stromeyer, sulphate of potash and sulphate of mag- 

 nesia is dissolved fi'om the polyhaUit of Ischl, by water, whilst sul- 

 phate of lime is left. According to Bauer, from the artificially 

 prepared combination of carbonate of potash and carbonate of 

 lime, water dissolves the first salt and leaves the last undis- 

 solved* ; whilst, according to Boussingault, the Gaylussite, oc- 

 curring in nature, which is similarly composed, withstands the 

 action of the water, and is only easily decomposed by it when it 

 has lost its Avater by being heatedf. 



Most of the other double salts, Hkewise composed of a salt 

 easy and of one difficult of solution, are not at all decomposed 

 by water. Common alum dissolves equally in water, without 

 the readily soluble sulphate of alumina being separated by it 

 from the sulphate of potash, which is of more difficult solution. 

 The bisulphate of potash, which must be considered as a double 

 salt, consisting of sulphate of potash and hydrate of sulphuric 

 acid, acts in a similar way towards water ; it also dissolves in 

 water without decomposition. But between the two examples of 

 double salts there is this difference, that, from the last salt alco- 

 hol separates the insoluble sulphate of potash, and dissolves the 

 hydi-ate of sulphuric acid, whilst the alum resists the decompo- 

 sition by aqueous alcohol, though the sulphate of alumina is 

 soluble, and the sulphate of potash insoluble, in it. 



The carbonate in the sesquicarbonate of the ammonia can 



• Poggendorff's Annalen, vol. xxiv. p. 367. \ Ibid. vol. vii. p. 99. 



