PROrKKDlNOS OK THK 



that thv brown carbon. uvous mat tors of the soil wore tbo priiui- 

 pul sourie of the carbon of plants, and tbat those substances 

 bein^ ronderoU soluble in water by the i)resence of alcaline and 

 other inorganic compounds in the soil, were thus brought 

 into a tit state to be absorbed by the r«)ots of jjlants. These two 

 theories have been well contrasted t(>i;etlier by Licl)i^, wiio has 

 shewn the many ^reat oljjcctions to the latter \ iew, and very 

 completely re-establislicd the old tlieory that ])lants derive their 

 carbon from the (lecoinposition of carbonic acid i::as ;uid not from 

 the absorj)tion of solid carbonaceous matters existing in the soil. 

 The clnef use then of the various organic substances added to 

 land, as far as they supply carbon, consists in their furnishing a 

 continual source of carbonic acid, a gas which is evolved by all 

 organic substances whilst undergoing decay. 



The importance of Nithookn in the growth of plants has in for- 

 mer times been greatly overlooked, and by some observers almost 

 entirely forgotten. It is true that Priestley and his contemporaries 

 thought it probable that this gas must have some influence on 

 vegetation, but their experiments were imperfect, and the con- 

 clusion which they appear to have arrived at was, that nitrogen 

 was of very little importance compared to carbon and oxygen. 

 More recently, as chemistry advanced, the assimilation of 

 nitrogen began to be thought more necessary to the growth of 

 plants, but still very indefinite ideas were entertained re- 

 specting the mode in which it was absorbed ; and although the 

 Agricultural Chemists at the commencement of this century were 

 aware that nitrates, and salts of ammonia had strong influence on 

 the growth of plants, and considered that they were valuable 

 chiefly as sources of nitrogen, yet they did not take into consi- 

 deration the fact that nitrogen can only be assimilated in a state 

 of combination and they were not aware that the absorption of 

 nitrogen by plants, was essential to the formation of those 

 compounds, on the presence of which the value of nearly all 

 vegetables and plants as articles of food mainly depends. Che- 

 mical enquiries into the compounds of nitrogen have shewn that 

 this singular element has as it were a reluctance to enter into 

 combination with other substances, not uniting with them under 

 ordinary circumstances when in the free state j but combining 

 with them for the most part easily, when in the nascent state, or 

 at the moment of being evolved from one of its compounds in a 

 state of decomposition. It is known for example that the 

 nitrogen of the air is unable under all ordinary circumstances 

 to unite with oxygen, carbon, and similar simple substances, and 

 from the knowledge of this we are led to conclude that plants 

 cannot derive their nitrogen from that great reservoir of free 



