170 ARRANGEMENT OF THE ELEMENTS IN COMPOUNDS* 



never happens with basic water. Thus starch or grape sugar, 

 in its ordinary hydrated state, consists of C 12 H 12 O 12 + 2HO ;- 

 of which the two atoms of water may be replaced by chloride of 

 sodium, and the compound formed, C }2 H 12 O 12 + 2NaCl. It 

 is to be observed that constitutional water is combined with a 

 salt rather than in it, and such an element is removed and 

 replaced without affecting the structure of the body to which 

 it may be said to be attached. The replacing substance may 

 also be a compound of a very different character from water, 

 for besides metallic oxides and salts, ammonia and certain 

 anhydrous acids appear to be capable of attaching themselves to 

 salts, in the same manner as constitutional water. 



Salts of the type of red chr ornate of potash. Several salts 

 unite with anhydrous acids. Thus both chloride of sodium 

 and chloride of potassium absorb and combine with two atoms 

 of anhydrous sulphuric acid without decomposition, when 

 exposed to the vapour of that substance. Sulphate of potash 

 also combines with one atom of anhydrous sulphuric acid. 

 All these compounds are destroyed by water. But the red 

 chromate of potash, generally called bichromate of potash, 

 which consists of chromate of potash together with one atom 

 of chromic acid, is possessed of greater stability, as is likewise 

 the compound of chloride of sodium or potassium with two 

 atoms chromic acid. The red chromate might be viewed as 

 a chromate of the chromate of potash, and the two last 

 compounds as bichromates of the chlorides of potassium and 

 sodium, but these expressions are more convenient than phi- 

 losophical, and it will be safer in the present state of our know- 

 ledge to assimilate these salts in composition to the combina- 

 tions of neutral bodies with constitutional water, particularly 

 as we find the proportion of acid to be variable, and generally 

 to be more than one atom. Thus a compound is known con- 

 taining one atom of potash and three of chromic acid, which 

 may be viewed as a combination of chromate of potash with 

 two atoms of chromic acid, and represented by KO, Cr O 3 

 + 2Cr O 3 . The red chromate of potash will then be KO, 

 Cr O 3 + Cr O 3 , and the chromate containing chloride of po- 

 tassium, KCl-f2CrO 3 . The biniodate of potash (iodate of 

 water and potash) may be rendered anhydrous, and while so, 

 is a salt of this class. 



Double Salts. Salts combine with each other, but by no 



