OS CHEMICAL NOMENCLATURE. 45 



we find the terms Injpointrlc acid and peroxide of nitrogen applied to the 

 fonrth oxide, the other oxides being called nitrous and nitric oxides, nitrous 

 and nitric acids. 



In the earlier edition of Fownes' ' Manual,' the terms protoxide and 

 binoxide of nitrogen are applied to the lower oxides, nitrons acid to the 

 third, hyponitric and nitric acids to the fourth and fifth. In the tenth 

 edition the terms nitrogen monoxide and dioxide are adopted for the two 

 lower and tetr oxide for the fourth oxide; the terms nitrous oxide and nitro- 

 gen trioxide are given to the third, and the terms nitric oxide and nitrogen 

 pentoxide to the fifth. The terms nitrous and nitric oxides here applied 

 to the fourth and fifth oxides had previously only been applied by chemists 

 to the fh'st and second. 



TABLES III.-V. 



The older chemists were agreed in designating the two oxides of 

 sulphur the sulphurous and the sulphuric acids respectively. In Fownes' 

 'Manual,' ed. 1863, the alternative names sulphur di- and trioxide are first 

 introduced, which, among the later writers, have gradually superseded 

 the former names. 



In a memoir in the ' Jahresbericht,' 1842, Berzelius recommends the 

 introduction of the names Di-, Tri- and Tetrathionic acids. This nomen- 

 clature has superseded the older names hyposulphuric, monosulphyposul- 

 phuric, &c, acids, though some few of the later writers retain the term 

 hyposulphuric acid. 



The discovery of the true hyposulphurous acid by Schiitzenberger 

 caused the acid, hitherto known by that name, to be designated thlosul- 

 phuric acid, as derived from sulphuric acid, by the replacement of one 

 atom of oxygen by sulphur. Hydrosulphurons acid, the name originally 

 proposed by Schiitzenberger for his acid, seems to be retained only by the 

 French writers. 



TABLES VI.-VII. 



The oxides of chromium afford an instance of change of names owing 

 to the discovery of another member of a series of compounds. Thus the 

 green oxide of chromium was designated the protoxide, until the isolation 

 of an oxide containing one atom of oxygen to one of the metal. The 

 latter compound was then called the protoxide, while the name of the 

 former was altered to sesquioxide. 



The potassium and lead salts of chromic acid afford a good example 

 of the want of unamimity of nomenclature among the older writers in 

 those cases in which there are derived from one acid two salts, the one 

 neutral, containing one equivalent of basic to one equivalent of acid oxide, 

 the others containing an excess of either of the oxides. Thus the acid or 

 red potassium chromate is called indifferently potassium di or bichromate, 

 but the former prefix is equally applied to the basic lead chromate. The 

 later writers have avoided this confusion of prefixes by introducing the 

 di or hi before the name of the acid or metal according as the salt con- 

 tains excess of the acid or the basic oxide, respectively, thus : — 



Potassium t7/chromate, but tf/plumbic chromate. 



