M8 nitrificatiopt. 



receives in tropical countries. It is there well known that too mnch 

 stirring of the soil is i 'ten prejudicial even in irrigated lands, where 

 consequently the bad i ifects cannot be attributed to too great a de- 

 gree of dryness. The information which has lately reached the 

 Academy of Sciences upon the agriculture, of the French posses- 

 sions in Africa, tend to make us perceive that the same cause pro- 

 duces the same effects in Algeria, and that it is not without reason 

 that the Arabs only work their lands that are preparing for grain 

 crops, very superficially. 



Humus is, in fact, dissipated by a process of slow combustion in 

 the air : in contact with oxygen, it produces carbonic acid, as is 

 proved by the experiments of M. de Saussure. Pure humus, moist- 

 ened with distilled water, confined in bell-glasses placed over me:»- 

 cury, formed carbonic acid, causing the disappearance of the oxygen 

 of the air. The volume of the acid gas formed, corresponded ii 

 volume with that of the oxygen which had disappeared. Humus, 

 therefore, in contact with air, gives off carbonic acid, and the phe- 

 nomenon here still takes place as if carbon were not alone consumed. 

 The loss experienced is greater than that which ought to occur from 

 the quantity of carbon which unites with the oxygen ; and Saussure 

 concluded that there is, at the same time, a loss of the elements of 

 water. The capital fact which results from these experiments of 

 Saussure, the deduction directly applicable to the theory of manures 

 is this : that humus is dissipated when it is exposed to the air, and 

 that during the slow combustion which it undergoes, it is a constant 

 source of carbonic acid gas. 



To complete the views that may throw light on the part played 

 by manures, I have still to speak of an important phenomenon which 

 occasionally takes place under the same conditions as those that ac- 

 company the decomposition, the putrefaction of animal matters : I 

 mean the spontaneous formation of nitric acid — the occurrence of 

 nitrification as it is called. Nitric acid results from the union of 

 azote with oxygen. Such at least is the constitution of this acid 

 when it is combined in salts ; but in its isolated state, it is always 

 united with a certain quantity of water. It has not yet been obtain- 

 ed, and it appears indeed not to exist, in the perfectly dry or anhy- 

 drous state. The azote, therefore, does not combine directly with 

 the oxygen ; there must be, at all events, the intervention of water, 

 and to effect the union of the two gases by means of the electric 

 spark, the mixture, according to Cavendish, must be moist. Never- 

 theless, the combination of azote with oxygen appears to be singular- 

 ly favored by the presence of earthy or alkaline bases, seeing that 

 in nature the nitrates are met with in a certain abundance ; but the 

 circumstances which determine their formation are still involved in 

 deep obscurity. 



Three distinct origins may be assigned to the natural nitrates : 

 Ist. certain soils, still indifferently studied, show an efflorescence of 

 nitrate of potash on their surface, or by lixiviation yield large quan- 

 tities of this salt. Such is the gource of the saltpetre which is im- 

 ported from India. 



