1/2 ARRANGEMENT OF THE ELEMENTS IN COMPOUNDS. 



supposed analogy is destroyed by what is known of the deriva- 

 tion of double salts. Sulphate of magnesia acquires an atom 

 of sulphate of potash in the place of an atom of water which is 

 strongly attached to it, in becoming the double sulphate of 

 magnesia and, potash. In the same way, the sulphate of water 

 has an atom of water also replaced by sulphate of potash, in 

 becoming the bisulphate of potash ; relations which appear in 

 the rational formulas of these salts : 



Sulphate of magnesia . . . . MgS(H)+6H 



Sulphate of magnesia and potash . . Mg S (K S)+6H 



Sulphate of water (acid of sp. gr. 1.78) . H S (H) 



Bisulphate of potash .... H S (K S) 



It thus appears that a provision exists in sulphate of magne- 

 sia itself for the formation of a double salt, and that the mole- 

 cular structure is unaltered, notwithstanding the assumption of 

 the sulphate of potash as a constituent. The derivation of the 

 acid oxalates likewise, throws much light on the nature of dou- 

 ble salts. The oxalate of potash contains an atom of constitu- 

 tional water, which is replaced by hydrated oxalic acid (the 

 crystallized oxalate of water), in the formation of the binoxalate 

 of potash (double oxalate of potash and water), or by the oxa- 

 late of copper in the formation of the double oxalate of potash 

 and copper, as exhibited in the following formulae, in which 

 the replacing substances are enclosed in brackets to mark them 

 as before : 



Oxalate of potash . . . . K CC (H) 



Binoxalate of potash . . . . K CC (H CC HJ 

 Oxalate of potash and copper . . K CC (Cu CC H 2 ) 



Now the anomalous salt, quadroxalate of potash, is derived in 

 the same way from the binoxalate, as the binoxalate itself is de- 

 rived from the neutral oxalate, two atoms of water being dis- 

 placed by two atoms of hydrated oxalic acid, thus : 



Binoxalate of potash . . K CC, H CC, (2H) 



Quadroxalate of potash . . K CC, HCC, (2HCCH 2 ) 



These examples illustrate the derivation of double salts by 

 substitution. The structure of the salts too exemplifies what 



