APPENDIX. 451 



are not mere mixtures. They have their own crystalline 

 form, and each particle of their crystals contanis all the ele- 

 ments of both. 



BINARY THEORY OP SALTS. Sulphate of potassa, and 

 other similar salts, are commonly regarded as ternary com- 

 pounds. But many chemists are of the opinion that they 

 are constituted after the plan of the binary salts, and their 

 acids on the plan of a hydrogen acid. They would write 

 sulphuric acid, SO4,H, instead of HO,SOs, thus indicating 

 that the hydrated acid is composed of the radical, S(X (a 

 compound playing the part of an element,) with hydrogen. 

 Sulphate of potassa would, according to this view, be writ- 

 ten K,SO4, instead of KO,SO3. The acid and salt are thus 

 represented as analogous in constitution to a hydracid and 

 a binary salt; thus, (SO4)H corresponds with C1H, and 

 K(SO4) with KC1. The advantage of this view is that it 

 makes but one great class of acids, and one of salts, associ- 

 ating substances which are analogous in their properties. 

 Hydrogen thus becomes characteristic of an acid. This view 

 also simplifies the subject of the production of salts from 

 acids, making it to consist simply in the replacement of the 

 hydrogen of the acid by a metal. Thus in the action of sul- 

 phuric acid (HO.SOa) on zinc, sulphate of zinc (ZnO,SOs) is 

 formed by the simple replacement of the hydrogen of the 

 acid by the metal zinc. As will be seen more clearly in 

 the introduction to Organic Chemistry, it is no conclusive 

 objection against the view, that the radical SO4 has not been 

 isolated. There is the best reason for believing in the exist- 

 ence of many such hypothetical radicals. A similar objection 

 has indeed been urged against the ordinary view, according 

 to which SOs neutralizes potassa in the sulphate of this base. 

 The objection lies in the fact that anhydrous sulphuric acid 

 is not possessed of acid properties, and can therefore be 

 scarcely regarded as an acid, in its anhydrous condition. 



