U2 NITRE CAVES. — NITRIC ACID FORMED IN THE SOIL. 



not to be recognized in sensible quantity, the production of this acid is 

 observed to proceed with a constant and steady pace. Thus, from the 

 walls of certain caves in Ceylon a layer is yearly pared off, which 

 yields an abundant crop of saltpetre (Dr. John Davy). The celebrated 

 Mammoth cave in Kentucky, situated in a limestone ridge, yields an 

 inexhaustible supply of nitrate of lime. During the war with Great 

 Britain, jfifiy men were constantly employed in lixiviating the earth of 

 this cave, and in about three years the washed earth is said to become 

 as strongly impregnated as at first. Through the cave a strong current 

 of air is continually rushing — inwards in winter, and outwards during 

 the summer months. On the plaster of old walls, especially in damp 

 situations, an efflorescence of this and other nitrates is frequently ob- 

 served over every part of Europe. In China, according to Davis, the 

 old plaster of the houses is so much esteemed as a manure, that parties 

 will often purchase it at the expense of a coating of new plaster. Old 

 clay walls, and especially the walls of clay-built huts, are said to be 

 very fertilizing to the land, when applied as a top-dressing, and in some 

 parts of England, where the land is poor, the people are said to pile up 

 the soil in the form of walls, in order to improve its quality. These lat- 

 ter facts seem to indicate that both in China and England nitric acid is 

 produced in similar circumstances, and that to its production the ferti- 

 lizing action of the old i)laster, and of the weathered clay, is alike to be 

 attributed. 



In the cultivated soil also, this acid is formed in ordinary circum- 

 stances. Braconnot found nitrate of potash in the botanic garden at 

 Nancy, in a portion of soil in which poj)pies [papaver somniferum) had 

 grown luxuriantly for ten years in succession — in larger quantity in the 

 soil surrounding the interlaced roots of an esclepias incarnata, growing 

 in an ordinary flower-pot, with a hole in the bottom — as well as in moss 

 earth, in which a plant of euphorbia hreoni had been grown in a pot. — 

 [Ann. de Chim. et de Phys., Ixxii., p. 33 to 35.] There is little reason 

 to doubt, indeed, that nitrates are to be found, in greater or less quantity, 

 in all cultivated soils. 



I shall not enter into a detailed inquiry how this nitric acid is formed. 

 It is probable that as in the atmosphere ammonia may be decomf)Osed 

 and give rise to the formation of nitric acid, so in the soil this acid may 

 result from a similar decomposition, proceeding more slowly, but accord- 

 ing to the same natural laws. In warm climates, indeed, it appears 

 certain that the ammonia which is evolved or formed during the decay 

 of animal and vegetable substances, does speedily, and to a great extent, 

 undergo oxidation,* and thus give rise to the greater abundance of nitric 

 acid with which the tropical soils abound. 



Thus, in the economy of nature, much ammonia is decomposed in the 

 soil also, and hence another cause for the constant diminution of the 

 quantity of this compound in addition to those already detailed in the 

 preceding section. 



But, besides the portion of this nitric acid, which owes its existence to 



* For the perfect oxidation cf 1 of ammonia, no less than S of oxygen are required. Thus 

 lof lof 3 of 



Ammonia. Nitric Acid. Wa 

 NH3 +80 = N05 + 3H' 



m 



