56 PROPERTIES IF NITRIC ACID. 



§ 6. Nitri': acid, its c^'utitution and 'properties. 



When the nitre or saltpetre of commerce is introduced into a retort, 

 covered with strong sulphuric acid (oil of vitriol*) and heated over a lamp 

 or a charcoal fire, red fumes are given off, and a transparent, often 

 brownish or reddish licjuid, distils over, which may be collected in a bot- 

 tle or other receiver of glass. This liquid is exceedingly acid and cor- 

 rosive. In small quantity it stains the skin and imparts a yellow colour 

 to animal and vegetable substances. In larger quantity it corrodes the 

 skin, producing a painful sore, rapidly destroys animal and vegetable 

 life, and speedily decomposes and oxidizesf all organic substances. 

 Being obtained from nitre, this liquid is called nitric acid. It consists of 

 nitrogen combined with oxygen, one equivalent of the former (N) being 

 united to 5 of the latter (O5), and is represented by NO5. 



This acid contains much oxygen, as its formula indicates, and its ac- 

 tion on nearly all organic substances depends upon the ease with which 

 it is decomposed, and may be made to part with a portion of this oxygen. 



In nature, it never occurs in a free state ; but it is found in many in- 

 tertropical (hot) countries in combination with potash, soda, and lime — in 

 the state of nitrates. It is an important character of these nitrates that, like 

 the salts of ammonia, they are all very soluble in water. Those of so- 

 da, lime, and magnesia attract moisture from the air, and in a damp at- 

 mosphere gradually assume the liquid form. 



Saltpetre is a compound of nitric acid with potash (nitrate of potash). 

 It is met with in the surface soil of many districts in Upper India, and 

 is separated by washing the soil and subsequently evaporating (or boil- 

 ing down) the clear li(|uid thus obtained. When pure, it does not be- 

 come moist on exposure to the air. It is chiefly used in the manufac- 

 ture of gunpowder, but has also been recommended and frequently and 

 successfully tried by the practical husbandman, as an influential agent 

 in promoting vegetation. 



In combination with soda, it is found in deposits of considerable thick- 

 ness in the district of Arica in Northern Peru, from whence it is im- 

 ported into tliis country, chiefly for the manufacture of nitric and sulphu- 

 ric acids. More recently its lower price has caused it to be extensively 

 employed in husbandry, especially as a top-dressing for grass lands. 

 Like the acid itself, these nitrates of potash and soda, when present in 

 large quantities, are injurious to vegetation. This is probably one cause 

 of the barrenness of the district of Arica in Peru, and of other countries, 

 where in consequence of the little rain that falls, the nitrous incrusta- 

 tions are accumulated upon the soil. In small quantity they appear to 

 exercise an important and salutary influence on the rapidity of growth, 

 and on the amount of produce of many of the cultivated grasses. This 

 salutary influence is to be ascribed, either in whole or in part, to the 

 constitution and nature of the nitric acid which these salts contain. It 



* Sulphuric acid is a compound of oxygen and sulphur, which is prepared by burning sul- 

 phur with certain precautions in large leaden chambers. It is also obtained directly by dis- 

 tilling ^een vitriol (sulphate of iron) at a high temperature in an iron still — hence its name oU 

 ofvitnol. It is a heavy, oily, acid, and remarkably corrosive liquid. In a concentrated state 

 it is exceedingly destructive both to animal and to vegetable life. 



t When a substance combines with oxygen^ either in consequence of exposure to the air 

 or in any other circumstances, . .8 said to become oxidized. 



