250 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



the second gives rise to a double decomposition. The first 

 does not decompose chlorides, and even combines with 

 some of them ; whereas the second decomposes them. . . . 



" The anhydrous and the hydrated acids, then, have not 

 any analogy with one another, and ought to be placed in 

 two perfectly distinct classes. . . . 



" The anhydrous and the hydrated sulphuric acid have 

 this in common : they both contain sulphur and oxygen ; 

 but with regard to analogy of properties, they do not 

 manifest any whatever" (Chemical Method, 44 and 45). 



Acids and acid anhydrides. At the close of his 

 " Chemical Method" (pp. 376 377) Laurent describes the 

 acidic oxides as ANHYDRIDES, derived from the acids 

 directly or indirectly by the removal of water. Laurent's 

 views on the nature of acids were soon accepted ; thus, in 

 1864, the following footnote was added to the third edition 



of Miller's " Elements of Chemistry" (Part II. p. 4) : 



. 



" The term acid has been employed by chemical writers 

 up to a late period, to designate indifferently either the 

 anhydrous bodies formed by the union of oxygen with the 

 non-metallic elements . . . (now more commonly called 

 anhydrides . . . ) or the hydrated compounds produced by 

 the action of water upon the anhydrides, such as ... 

 oil of vitriol or sulphuric acid. To avoid this confusion, 

 produced by the application of the same term to two 

 substances essentially distinct, it will be convenient to follow 

 the practice of many later authors, and to limit the term 

 acid to those hydrated bodies which are really salts of 

 hydrogen." 



The system of nomenclature introduced by Laurent has 

 persisted to the present day. The oxides of sulphur are 

 described as SULPHUROUS ANHYDRIDE and SULPHURIC 

 ANHYDRIDE, whilst the names SULPHUROUS ACID and SUL- 

 PHURIC ACID are restricted to the hydrated acids formed 

 by combining the oxides with water to form hydrogen sulphite 

 and hydrogen sulphate respectively. So too the acidic oxide 



