AGRICULTURAL CHEMISTRY. 139 



This striking experiment demonstrates that nitric acid directly 

 serves to supply nitrogen to plants. In fact, it appears to equal 

 ammonia in its assimilability. 



Liebig was formerly of the opinion that ammonia was the only form 

 in which vegetation could be supplied with nitrogen, and that nitric 

 acid was not appropriated by the plant until after it had become con- 

 verted into ammonia in the soil. We know that under the influence 

 of certain bodies having strong affinities for oxygen, nitric acid is 

 transformed into ammonia, hydrogen displacing oxygen. This change 

 was supposed to occur in the soil by virtue of the action of the car- 

 bonaceous matters (humus) there present. Now, while this may 

 actually happen under certain circumstances, it is well ascertained 

 that the soil and natural waters more generally contain nitrates than 

 salts of ammonia, and the actual conversion of ammonia into nitrates 

 in the soil has been experimentally traced. 



The presence of nitrous oxyd in the atmosphere is not as yet directly 

 proved, from want of a proper method of detecting it when forming 

 but a small proportion of a gaseous mixture; but Krhop has shown the 

 probability of its occurrence there, and has proved that it may serve 

 as a source of nitrogen to plants. What may be its significance in the 

 actual nourishment of vegetation remains to be determined. 



The important questions now arise, what are the sources of the 

 water, carbonic acid, ammonia, and nitric acid, that exist in the atmos- 

 phere ? are these minute quantities liable to exhaustion ? are they 

 sufficient to supply vegetation with carbon, hydrogen and nitrogen? 



The time was — so the reasonings of geology convince us — when the 

 soil, having scarcely cooled down from a state of fusion by fire, could 

 contain no carbon, or at least no nitrogen, in a form capable of feeding 

 plants. Consequently, at this period all the nitrogen, and by far the 

 larger share of the carbon, destined to aid the growth of plants must 

 have existed in the air: and although processes subsequently came 

 into operation Avhereby portions of these substances were incorporated 

 with the soil, the final result of natural operations is to restore them 

 in great part to the atmosphere. 



The effect of oxygen, as manifested in the processes of decay, com- 

 bustion, and animal nutrition, is to bring down the vegetable organism 

 to the inorganic level — to convert the carbo-hydrates, the albuminoids 

 and other proximate principles of vegetation, into carbonic acid, water, 

 ammonia, and nitric acid, the very materials out of which, under the 

 influence of the vital principle, they were constructed. 



These three varieties of chemical disorganization, which were par- 

 allel with the vital up-building of vegetation, deserve a 'somewhat 

 extended notice. 



Decay is a general term expressing the wasting or destruction of 

 organic bodies under the influence of warmth, oxygen, and water. 



The carbo-hydrates, when perfectly pure and dry, may be preserved 

 indefinitely without undergoing any change. This we know in the 

 case of cellulose, (cotton, paper,) sugar, starch, &c. In presence of 

 a certain amount of water, and exposed to the air at a warm tempera- 

 ture, they undergo change; which, in case of cellulose, is very slow, 



