HISTORICAL AND INTRODUCTORY 15 



awake pity." Liebig's ridicule did what neither de Saussure's 

 nor Boussingault's logic had done : it finally killed the humus 

 theory. Only the boldest would have ventured after this to 

 assert that plants derive their carbon from any source other 

 than carbon dioxide, although it must be admitted that we have 

 no proof that plants really do obtain all their carbon in this 

 way. Thirty years later, in fact, Grandeau (112) adduced 

 evidence that humus may, after all, contribute something to 

 the carbon supply, and his view still finds acceptance in 

 France ; l for this also, however, convincing proof is lacking. 

 But for the time carbon dioxide was considered to be the sole 

 source of the carbon of plants. Hydrogen and oxygen came 

 from water, and nitrogen from ammonia. Certain mineral 

 substances were essential : alkalis were needed for neutrali- 

 sation of the acids made 1 by plants in the course of their vital 

 processes, phosphates were necessary for seed formation, and 

 potassium silicates for the development of grasses and cereals. 

 The evidence lay in the composition of the ash : plants might 

 absorb anything soluble from the soil, but they excreted from 

 their roots whatever was non-essential. The fact of a sub- 

 stance being present was therefore sufficient proof of its 

 necessity. 



Plants, Liebig argued, have an inexhaustible supply of 

 carbonic acid in the air. But time is saved in the early stages 

 of plant growth if carbonic acid is being generated in the soil, 

 for it enters the plant root and affords extra nutriment over 

 and above what the small leaves are taking in. Hence a 

 supply of humus, which continuously yields carbonic acid, is 

 advantageous. Further, the carbonic acid attacks and dissolves 

 some of the alkali compounds of the soil and thus increases 

 the mineral food supply. The true function of humus is to 

 evolve carbonic acid 



The alkali compounds of the soil are not all equally soluble. 

 A weathering process has to go on, which is facilitated by 

 liming and cultivation, whereby the comparatively insoluble 



1 See e.g. L. Cailletet (64), Jules Lefevre (169), and J. Laurent, Rev. gen, 

 hot., 1904, 16, 14. 



