6 THE FERTILITY OF THE SOIL 



pointed out that it must be this salt which feeds the 

 plant, because none can be extracted from soils in 

 which plants are growing. So general had this as- 

 sociation of nitre with the fertility of soils become, that 

 in 1675 JOHN EVELYN, of this college, wrote ' / firmly 

 believe that where saltpetre can be obtained in plenty we 

 should need to find other composts to ameliorate our ground' ; 

 and HENSHAW, of University College, one of the first 

 members of the Royal Society, wrote ' / am convinced 

 indeed, that the salt which is found in vegetable and animals 

 is but the nitre which is so universally diffused through all 

 the elements, (and must therefore make the chief ingredient 

 in their nutriment, and by consequence all their generation] 

 a little altered from its first complexion! But these 

 promising beginnings of the theory of plant nutrition 

 came to nothing ; the Oxford movement in the seven- 

 teenth century was not the real dawn of the science. 

 At the end of the century the human mind, which had 

 sought out of doors for some relief from the fierce 

 religious controversies with which it had been so long 

 engrossed, turned indoors again and went to sleep for 

 another century. MAYOW'S work was forgotten, and it 

 was not until PRIESTLEY and LAVOISIER, DE SAUSSURE, 

 and others, about the beginning of the nineteenth century, 

 arrived at a sound idea of what air really was and what 

 its properties were, that it became possible to build 

 up afresh a sound theory. By about 1840 it had been 

 definitely settled what the plant was composed of, and 

 whence it derived its nutriment : the carbon compounds 

 which constitute nine-tenths of the dry weight (from the 

 air), the nitrogen and the ash (from the soil). LIEBIG 

 had contributed little to the realms of discovery, but his 

 brilliant theories, and the weight of his authority, had 



