92 CHEMISTR V FOR A GRIC UL TURAL STUDENTS 



warm weather, and thus crops are provided with combined nitrogen in the 

 form that plants most easily assimilate. The nitrate of soda (Chili salt- 

 petre) occurring as a deposit in the rainless districts of South America, 

 and which is now so largely employed (together with sulphate of ammonia) 

 as a nitrogenous manure, owes its origin, no doubt, to the same cause. 



Another very important application of these processes is the natural puri- 

 fication of water. When water has become contaminated with sewage, the 

 nitrogenous organic matter serves as a food for disease germs which, under 

 favourable conditions, multiply with enormous rapidity. But putrefaction 

 of the organic matter and the nitrification of the ammonia produced 

 deprives such germs of the means of development. In the analysis of 

 drinking waters determinations are made of (i) the amount of total organic 

 matter (as indicated by the quantity of permanganate of potassium it reduces, 

 and therefore decolorises in presence of acid — see p. 84) expressed as 

 "oxygen absorbed"; (2) the amount of nitrogenous organic matter 

 expressed as "albuminoid ammonia"; (3) the amount of "free am- 

 monia"; (4) the amount of "nitrates"; and (5) the amount of 

 chlorides expressed as "chlorine." Should the albuminoid ammonia 

 be high, recent sewage contamination is indicated, and the water would 

 be condemned for drinking purposes. Should the free ammonia be 

 high, the water would also be regarded with suspicion ; but should the 

 albuminoid and free ammonia be low, and the nitrates high, it indicates 

 that sewage contamination has at some time occurred, but that sufiicient 

 time has passed for putrefaction and nitrification to go on, so that the water 

 need not necessarily be condemned. The presence of much chlorine is 

 suspicious, because common salt is an essential element of animal food, and 

 therefore occurs in sewage, but the salt may be derived from other sources. 

 The oxygen absorbed will be high if the organic matter be high, hence 

 potassium permanganate provides a rough test for the purity of water from 

 organic contamination. (Organic matter, however, is not necessarily highly 

 nitrogenous.) Acidify a cylinder full of the water with a few drops of pure 

 sulphuric acid, and then add a weak solution of permanganate, a drop at a 

 time, until it is no longer decolorised after standing. Compare distilled 

 water with tap water and rain water. Chlorides may be tested for with 

 nitric acid and nitrate of silver, and nitrates with sulphuric acid and indigo 

 solution. 



The putrefaction of organic matter and the nitrification of ammonia are 

 fermentative changes due to the action of micro-organisms. Among the 

 conditions favourable to both kinds of fermentation are sufficient moisture, 

 sufficient air, and sufficient warmth. That air must be necessary to nitri- 

 fication is obvious when it is remembered that the conversion of ammonia 

 into nitric acid is a process of oxidation, and since nitrate is the form of 



