TRANSACTIONS OF SECTION B. 511 



With regard to the nitrogen of vegetation, both from the known characters of 

 free nitrogen, and as he considered a legitimate deduction from direct experiments, 

 he aro-ued that pLants did not take up free or uncomhined nitrogen, either from the 

 atmosphere, or dissolved in water and so absorbed by the roots. The source of the 

 nitroi^en of vegetation was, he maintained, ammonia ; the product of the putrefaction 

 of one generation of plants and animals aflbrding a supply for its successors. He 

 pointed out that, in the case of a farm receiving nothing from external som-ces, and 

 selling off certain products, the amount of nitrogen in the manure derived by the con- 

 sumption of some of the vegetable produce on the farm itself, together with that due 

 to the refuse of the crops, must always be less than was contained in the crops 

 o-rown ; and he concluded that though the quantity so returned to the land was 

 important, a main source of the nitrogen assimilated over a given area was that 

 brought down from the atmosphere in rain. 



There can be no doubt that, owing to the limited and defective experimental 

 evidence then at command ou the point, Liebig at that time (as he has since) 

 o-reatly over-estimated the amount of ammonia available to vegetation from that 

 soiu'ce. In Boussiugault's reclnmation already referred to, he gave much more 

 prominence to the importance of the nitrogen of manures. In Liebig's next edition 

 (in 1843) he combated the notion of the relative importance of the nitrogen of 

 manures ; maintained, in opposition to the view put forward in his former edition, 

 that the atmosphere afforded a sufficient supply of nitrogen for cultivated as well as 

 for uncultivated plants ; that the supply was sufficient for the cereals as well as for 

 len-uminous plants ; that it was not necessary to supply nitrogen to the former ; 

 and he insisted very much more strongly than formerly on the relative importance 

 of the supply of the incombustible, or, as he designated them, the ' inorganic ' or 

 'mineral,' constituents. 



As to the incombustible or mineral constituents themselves, Liebig adduced 

 many illustrations in proof of their essentialness. He called attention to the 

 variation in the composition of the ash of plants grown on ditfereut soils ; and he 

 assumed a greater degree of mutual replaceability of one base by another, or of one 

 acid by another, than could be now admitted. He traced the difference in the 

 ■mineral composition of different soils to that of the rocks which had been their 

 source ; and he seems to have been led by the consideration of the gradual action 

 of ' wetttJieri7if/,' in rendering available the otherwise locked-up stores, to attribute 

 the beneiits of fallow exclusively to the increased supply of the incombustible 

 constituents which would, by its agency, be brought into a condition in which they 

 could be taken up by plants. 



The benefits of an alternation of crops Liebig considered to be in part explained 

 by the influence of the excreted matters from one description of crop upon the 

 o-rowth of another. He did not attach weight to the assumption that such matters 

 would be directly injurious to the same description of crop ; but he supposed rather 

 that the matters excreted were those which the plant did not need, and would 

 therefore be of no avail to the same description of plant, but would be of use to 

 another. He, however, attributed much of the benefits of a rotation to different 

 mineral constituents being required from the soil by the respective crops. 



Treating of manure, he laid the greatest stress on the return b}' it of the potass 

 and the phosphates removed by the crops. But he also insisted on the importance 

 of the nitrogen, especially that in the liquid excretions of animals, and condemned 

 the methods of treatment of animal manures by which the ammonia was allowed 

 to be lost by evaporation. It is curious and significant, however, that some of the 

 passages in his first edition, in which he the most forcibly urges the value of the 

 nitrogen of animal manures, are omitted in the third and fourth editions. 



The discussion of the processes of fermentation, decay, and putrefaction, and 

 that of jwisons, contagions, and miasms, constituted a remarkable and important 

 part of Liebig's first report. It was the portion relating to poisons, contagions, 

 and miasms that he presented to this Section as an instalment, at the meeting of 

 the Association held at Glasgow in 1840. It was in the chapters relating to the 

 several subjects here enumerated that he developed so prominently his views on 

 the influence of contact in inducing chemical changes. He cited many known 



