March 1, 1889.] 



♦ KNOWLEDGE ♦ 



101 



a large proportion of chalky matter, and is rich in the remains 

 of luxuriant vegetation, these nitrates are produced in abund- 

 ance, and the soil near the surface is full of them. Frequently 

 deposits of nitre are found in those places, lookinglike hoar frost 

 on the surface of the ground. In Chili similar conditions have 

 existed, and the deposits being near the coast, it is probable 

 that they are largely derived from animal matter, most 

 probably marine animals and birds ; in fact, remains of 

 ancient shells indicate that the sea was there in days gone 

 by, although the deposits are now much above the sea-level. 

 Even in England, where the climatic conditions are such 

 that the little organisms cannot thrive so as to work with 

 vigour all the year round, yet on a good arable soil the 

 amount of organic nitrogen converted into nitric acid or 

 nitrates during a year of fallow in a good season may be 

 put down at near 100 pounds of nitrogen per acre, which is 

 equivalent to about 600 pounds of Chili saltpetre. 



In order to extract the nitrates the saltpetre-earth 

 is soaked in water ; the resulting liquid contains all the 

 nitrates mixed with various other soluble constituents. In 

 Chili this liquid is evaporated down ; it then deposits an 

 impure sodium nitrate mixed with varying, and fiequently 

 very large, percentages of sodium chloiide (common salt) 

 and sodium suljshate, and with smaller quantities of calcium 

 and magnesium salts. This impure nitrate, known as crude 

 nitre or " caliche," is refined by re[)eatedly redissolving in 

 water and allowing it to crystallise, but a little common 

 salt generally remains attached to the crystals of cubic nitre, 

 so that the sodium nitrate of commerce as a rule retains 

 about 2 f)er cent, of salt. In India and elsewhere two kinds 

 of saltpetre-earth exist : 1. Earth licli in potash, which is 

 treated in the manner just described, and yields impure 

 potassium nitrate. 2. Earth containing only small quanti- 

 ties of potash, but mvich lime and magnesia ; when this is 

 treated with water the corresponding nitrates pass into 

 solution, and the liquid is therefore mixed with crude potash 

 (wood-ashes, containing a large proportion of potassium 

 carbonate), or the earth is mixed with wood-ashes before 

 being soaked in water. The resulting liquid then leaves on 

 evaporation impure potassium nitrate. Impure potassium 

 nitrate, crude saltpetre, or " grough," contains potassium and 

 sodium chorides, sometimes in large quantities, and organic 

 matter. It is refined very much in the same manner as the 

 Chili saltpetre. 



Pure potassium nitrate forms long six- sided clear crystals, 

 which fuse readily, and, if not heated to a red heat, leave on 

 cooling a white opaque fibrous mass known as sal prunella. 

 Pure sodium nitrate forms crystals which look like cubes, 

 hence it is sometimes called cubic nitre. Both nitres give up 

 oxygen on heating to redness, and both, especially potassium 

 nitrate, cause rapid combustion in contact with hot com- 

 bustibles, and also with many metals. It is on account of its 

 comparatively moderate activity in such combustion, and a 

 tendency to absorb moisture from the air, that sodium nitrate 

 is unfit for the manufacture of gunpowder, for which pur- 

 pose such large quantities of potassium uiti-ate are utilised. 

 Sodium nitrate is, however, employed in agriculture, for the 

 preparation of potassium nitrate, for the production of 

 nitric acid, and in the manufacture of sulphuric acid (oil of 

 vitriol). 



Its employment in agriculture is, without doubt, the 

 most importa,nt industrial application of nitrate of soda. 

 It is a well-known fact that plants, like animals, require con- 

 siderable quantities of nitrogen for their proper development, 

 and therefore, if plants are to develop well, the soil must 

 contain this necessary constitvient, not only in a form suit- 

 able for plants to assimilate, but also in sufficient quantities 

 to supply their requirements. Tlio following table shows 

 what these requirements ai-e for ordinary crops. In the first 



column is the name of the crop, in the second the average 

 weight of the total crop at harvest, in the third the 

 average amount of nitrogen contained in such a crop. The 

 numbers are taken from Warington's " Chemistry of the 

 Farm," and are per acre : — 



These numbers demonstrate what large quantities of 

 nitrogen the various crops remove from the soil, whilst the 

 following table, compiled from various papers by Sir John 

 Lawes and Dr. Gilbert, referring to results obtained on the 

 clay soil of the Rothamsted Farm, shows the effect of trying 

 to grow crops with and without nitrogenous manure on a 

 soil impoverished by the growth of previous crops, and con- 

 sequently poor in nitrogen : — 



The first table shows the requirements of various crops, the 

 second illustrates the advantage derived bj' the employment 

 of nitrogenous manures. Although, on most farms, much 

 nitrogen is generally returned to the soil as farmyard manure, 

 there is even then a large deficit which has to be made up by 

 the application of artificial manures, and for this purpose many 

 substances are sold ; but, of all of them, nitrate of soda and 

 ammonium salts are the best, for the simple reason that the 

 others have generally to go through a preliminary period of 

 change before all their nitrogen becomes available for plant 

 nutriment, whereas the nitrogen of nitrate of soda is quite 

 ready for the immediate use of the plant ; and ammoniacal 

 nitrogen, if not alreadj' available, very rapidly becomes so. 



The results of numerous comparative experiments indicate 

 that, on the whole, nitrate of soda is superior to the 

 ammonium salts as a source of nitrogen for plants, but, 

 excellent manure a.s it is, its use cjvn become otherwise than 

 beneficial if it be injudiciously applied. 



Pure nitrate of soda contains in every 100 lbs. about 27 lbs. 

 of sodium, 16.V lbs. of nitrogen, and 56,V lbs. of oxygen ; but 

 owing to the impurities the nitrate of soda of commerce has 

 only about 15i lbs. of nitrogen. The nitrogen is the active con- 

 stituent, and the only one that needs to be taken into consider- 

 ation when calculating quantities required as dressings for 

 crops. Nitrate should be used only in such quantity as we know 

 by experience is sufficient to supply enough nitrogen for a 

 profitable increase of crop, any fui-ther addition being money 



