HISTORICAL AND INTRODUCTORY 7 



bon dioxide in small quantities was a vital necessity for plants, and they 

 perished if it was artificially removed from the air. It furnished them 

 not only with carbon, but also with some oxygen. Water is also 

 decomposed and fixed by plants. On comparing the amount of dry 

 matter gained from these sources with the amount of material that can 

 enter through the roots even under the most favourable conditions, he 

 concludes that the soil furnished only a very small part of the plant 

 food. Small as it is, however, this part is indispensable : it supplies 

 nitrogen — une partie essentielle des vdgitaux — which, as he had shown, 

 was not assimilated direct from the air ; and also ash constituents, qui 

 peuvent coniribuer d former, comme dans les animaujc, leur parties solides 

 ou osseuses. Further he shows that the root is not a mere filter allow- 

 ing any and every liquid to enter the plant ; it has a special action and 

 takes in water more readily than dissolved matter, thus effecting a 

 concentration of the solution surrounding it ; different salts, also, are 

 absorbed to a different extent. Passing next to the composition of the 

 plant ash, he shows that it is not constant, but varies with the nature 

 of the soil and the age of the plant ; it consists mainly, however, 

 of alkalis and phosphates. All the constituents of the ash occur in 

 humus. If a plant is grown from seed in water there is no gain in ash : 

 the amount found at the end of the plant's growth is the same as was 

 present in the seed excepting for a relatively small amount falling on 

 the plant as dust. Thus he disposes finally of the idea that the plant 

 generated potash. 



After the somewhat lengthy and often wearisome works of the ear- 

 lier writers it is very refreshing to turn to de Saussure's concise and 

 logical arguments and the ample verification he gives at every stage. 

 But for years his teachings were not accepted, nor were his methods 

 followed. 



Between 1802 and 181 2 Davy gave annually some lectures on 

 agricultural chemistry, which were published in 1813 (79), and form 

 the earliest text-book of the modern period. Whilst no great advance 

 was made by Davy himself (indeed his views are distinctly behind 

 those of de Saussure) he carefully sifted the facts and hypotheses of 

 previous writers, and gives us an account, which, however defective in 

 places, represents the best accepted knowledge of the time, set out in 

 the new chemical language. He does not accept de Saussure's con- 

 clusion that plants obtain their carbon chiefly from the carbonic acid of 

 the air : some plants, he says, appear to be supplied with carbon chiefly 

 from this source, but in general he supposes the carbon to be taken in 

 through the roots. Oils are good manures because of the carbon and 



