INTRODUCTION. 5 



cess of burning, their lime excepted, and the fact of their 

 being taken from' the soil and constituting the indispen- 

 sable food of plants, his Lordship was unacquainted with. 

 The gist of agricultural chemistry with him was, that 

 plants are "composed of gases with a small proportion of 

 calcareous matter;" for "although this discovery may 

 appear to be of small moment to the practical farmer, yet 

 it is well deserving of his attention and notice, as it 

 throws gi-eat light on the nature and food of vegetables." 

 The fact being then known that plants absorb carbonic 

 acid from the air, and employ its carbon in their growth, 

 the theory was held that fertilteers operate by promoting 

 the conversion of the organic matter of the soil or of 

 composts into gases, or into soluble humus, which were 

 considered to be the food of plants. 



The first accurate analysis of a vegetable substance was* 

 not accomplished until fifteen years after the publication 

 of Dnndonald's Treatise, and another like period passed 

 before the means of rapidly multiplying good analyses 

 had been worked out by Liebig. So late as 1838, the Got- 

 tingen Academy offered a prize for a satisfactory solution 

 Of the then vexed question whether the ingredients of 

 ashes are essential to vegetable growth. It is, in fact, 

 during the last fifty years that agricultural chemistry has 

 come to rest on sure foundations. Our knowledge of the 

 structure and physiology of plants is' of like recent devel- 

 opment. What immense practical benefit the farmer has 

 gathered from this advance of science ! Chemistry has 

 ascertained what vegetation absolutely demands for its 

 growth, and points out a multitude of sources whence 

 the requisite materials for crops can be derived. Cato 

 and Columella knew indeed that ashes, bones, bird- 

 dung and green manuring, as well as drainage and aera- 

 tion of the soil, were good for crops ; but that carbonic 

 acid, potash, phosphate of lime, and compounds of nitro- 

 gen are the chief pabulum of vegetation, they did not 



