CONSTITUTION OF SALTS. I7.i 



may be called consecutive combination. The basis of the last 

 mentioned salt, for instance, is oxalate of potash, which is in 

 direct combination with oxalate of water. A compound body 

 is thus produced which seems to unite as a whole with two 

 atoms of hydrated oxalic acid. This is very different from the 

 direct combination of all the elements which compose the salt. 



In the formation of other classes of double salts, no substi- 

 tution is observed, but simply the attachment of two salts to- 

 gether, often of an anhydrous with a hydrated salt, in which 

 case the last often carries its combined water along with 

 it, and sometimes acquires an additional proportion. Thus 

 in the formula of the double chloride of potassium and cop- 

 per, K Cl + Cu Cl, 2HO, the formulae of its constituent salts 

 reappear without alteration ; and in that of alum, sulphate of 

 potash is found with the hydrated sulphate of alumina annexed, 

 of which the water is increased from eighteen to twenty-four 

 atoms. In these and all other double salts, the characters of 

 the constituent salts are very little affected by their state of 

 union. If one of them has an acid reaction, like sulphate of 

 alumina or chloride of copper, it retains the same character in 

 combination ; and nothing resembling a mutual neutralization 

 of the salts by each other is ever observed. 



The compounds of chlorides with chlorides, and of iodides 

 with iodides are numerous, and were viewed by Bonsdorf as 

 simple salts, in which one of the chlorides is the acid, and the 

 other the base. But such an opinion can no longer be enter- 

 tained, the chlorides themselves being unquestionably salts, 

 and their compounds, therefore, double salts. 



The combinations of such salts with each other as contain 

 different acids are not so well understood, the theory of their 

 formation having been little attended to. They are in general 

 decomposed by water, and easily if the solubility of one of their 

 constituents is considerable, as is observed of the compounds of 

 iodate of soda with one and with two proportions of chloride of 

 sodium, of the biniodate of potash with the sulphate of potash, 

 of the oxalate of lime with the chloride of calcium. 



The compound cyanides which form a considerable class of 

 salts must be excepted from all the preceding general statements 

 in regard to double salts. Cyanides of the same family combine 

 together, as cyanide of iron with cyanide of hydrogen; the 

 compound cyanide also generally consists of three and not of 



