208 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



and Lavoisier. The strong acid from which saltpetre is 

 derived was distinguished in 1787 as nitric acid and its salts 

 as nitrates ; the weak acid, present in ignited saltpetre, was 

 distinguished as nitroics acid and its salts as nitrites ; the name 

 nitrous acid was, however, also applied to nitrogen peroxide 

 and to the red fuming variety of nitric acid. Typical formulae 

 are as follows : 



/Nitric acid, HNO 3 ) f Saltpetre (potassium nitrate), KNO 3 \ 

 \Nitrous acid, HNO 2 /\ Ignited ( nitrite), KNO 2 J 



/Silver nitrate, AgNO 3 (soluble in water) | 

 \Silver nitrite, AgNO 2 (sparingly soluble)] 



Lavoisier, in 1776, attempted to analyse strong nitric acid 

 with the help of mercury, as in the case of sulphuric acid (p. 168). 

 He also synthesised a weak acid by mixing nitric oxide and 

 oxygen over water. Cavendish, in 1781, showed that the latter 

 process gives both the ordinary (nitric) and the phlogisticated 

 (nitrous) acid. 



Gay-Lussac, in 1816, finally determined the composition of 

 the two acids by mixing nitric oxide and oxygen : (a) over potash, 

 producing a pure nitrite ; (d) in a narrow tube over water, pro- 

 ducing pure nitric acid. 



The action is shown by the following equations : 



(a) 



t,vols, i vol. (Potassium 



nitrite.) 



4 vols. 3 vols. (Nitric acid.) 



Since nitric oxide contains half its volume of nitrogen and 

 half its volume of oxygen, the four volumes of nitric oxide 

 shown in each of the above equations contain two volumes of 

 nitrogen and two volumes of oxygen ; the anhydrides of the 

 two acids therefore have the following composition : 



Nitrous acid Nitric oxide : oxygen = $ i 



Nitrogen : oxygen = 2 3 



Nitric acid Nitric oxide : oxygen ^ 3 



Nitrogen : oxygen = 2 5. 



