I.] SCIENCE AND TRADITION 5 



of a farming tradition and passed more or less into the 

 common routine. Even at the present time there are 

 many beliefs and practices more or less current among 

 farmers, which science has neither verified nor disproved, 

 and which may either be examples of sound observation 

 or only imperfect generalisations. Such opinions 

 require to be examined with the utmost care and open- 

 mindedness, for even when correct they are of no final 

 use to agriculture until they have been explained and 

 absorbed into the general stream of scientific knowledge. 

 The value of many fertilisers must have been observed 

 and lost sight of over and over again, because of the 

 lack of any general theory to serve as a touchstone and 

 discriminate between the true and the false reports. 

 So, despite the experience that was accumulating 

 respecting the fertilising value of this or that substance, 

 no real progress towards a theory of manuring was 

 made until the close of the eighteenth and the 

 beginning of the nineteenth century. 



Before the development of a science of chemistry it 

 was naturally impossible to form any idea of how a 

 plant came to grow ; while the nature of the plant 

 itself, of the air, water, and earth were equally unknown, 

 no correct opinion could be reached as to how the latter 

 gave rise to the former. In spite of Palissy's very 

 sound conclusions as to the salts plants draw from the 

 ground, Van Helmont described an experiment to 

 show that a tree is made out of water alone. Jethro 

 Tull, arguing from his hoeing experiments, concluded 

 that manures were unnecessary : for the soil, if only 

 stirred up enough and exposed to the air, will provide 

 all that the plant requires. Even so late as 1810 we find 

 Thaer writing that there is no doubt that the fallow 

 absorbs or attracts the fertilising properties of the 

 atmosphere. 



