2 SOILS AND FERTILIZERS 



1 



weighing 5 pounds, which he duly watered with rain 

 and distilled water. After five years he pulled up the 

 willow and it now weighed 169 pounds and 3 ounces." 1 

 He concluded that 164 pounds of roots, bark, leaves, 

 and branches had been produced by direct transmu- 

 tation of the water. 



It is evident from the preceding example that any- 

 thing like an adequate idea of the growth and compo- 

 sition of plant bodies could not be gained until the 

 composition of air and water was established. 



The discovery of oxygen by Priestley in 1774, of 

 the composition of water by Cavendish in 1781, and 

 of the r61e which carbon dioxide plays in plant and 

 animal life by De Saussure and others in 1800, formed 

 the nucleus of our present knowledge regarding the 

 sources of matter stored up in plants. It was between 

 1760 and 1800 that alchemy lost its grip because of 

 advances in knowledge and the way was opened for 

 the development of modern chemistry. 



De Saussure's " Recherches sur la Vegetation," pub- 

 lished in 1804, was the first systematic work showing 

 the sources of the compounds stored up in plant bodies. 

 He demonstrated, quantitatively, that the increase in the 

 amount of carbon, hydrogen, and oxygen, when plants 

 were exposed to sunlight, was at the expense of the 

 carbon dioxide of the air, and of the water of the soil. 

 He also maintained that the mineral elements derived 

 from the soil were essential for plant growth, and gave 

 the results of the analyses of many plant ashes. He 



