NITRIC ACID AND THE NITRATES 91 



Among these is ferrous sulphate, which is oxidised by this 

 means to ferric salt. In this case nitric oxide is the product of 

 reduction, and as this combines with the excess of ferrous 

 sulphate to form a black compound, the reaction serves as a 

 test for nitric acid and the nitrates."* To a very weak solution 

 of saltpetre in a test-tube add a crystal of ferrous sulphate and 

 allow to dissolve. Then very carefully, down the side of the 

 tube, pour a few drops of pure strong sulphuric acid, so that it 

 forms a layer at the bottom of the liquid. Note that a black 

 ring is formed. 



Another substance that reduces, and is oxidised by, nitric 

 acid is indigo. To the very weak solution of saltpetre (a solu- 

 tion of I in 10,000 is quite strong enough) add an equal volume 

 of pure, strong sulphuric acid, and, while the liquid is still hot, 

 add a solution of indigo in sulphuric acid drop by drop. Note 

 that the indigo is decolorised. The amount of indigo that 

 can be decolorised serves as a rough comparative test for the 

 quantity of nitrate present in two solutions. Test some dis- 

 tilled water, tap water, and the drainage water from arable 

 land for nitrates by both methods. 



All nitrates are soluble in water, so that there is no precipi- 

 tant that serves as a test for nitric acid and the nitrates. 



Putrefaction and Nitrification. — Saltpetre or nitre, the nitrate of 

 potassium, is found as an efflorescence on the soil round villages in hot 

 countries, due to the putrefaction of nitrogenous organic matter, the nilri- 

 fication of the ammonia thus produced, and the combination of the nitric 

 acid formed with the potash of the soil. By washing the soil with water 

 the nitre is dissolved out, and may be obtained by evaporating the solution. 

 The same series of changes may occur in a stable, where a strong smell of 

 ammonia, due to the putrefaction of the manure, is sometimes observed, 

 and crystals of nitrate of calcium found adhering to the mortar of the walls. 

 In cultivated soil, too, these changes are constantly going on, especially in 



\6FeSO4 -h 3II2SO4 + zO = sFe^sSOj + 3H0O 

 FeS04-l-NO = FeS04.NO 



