2 SOILS AND FERTILIZERS 



and distilled water. After five years he pulled up the 

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

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

 and branches had been produced by the direct trans- 

 mutation 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 Priestly, in 1774, of 

 the composition of water by Cavendish in 1781, and 

 of the role which carbon dioxide plays in plant and 

 animal life by DeSaussure and others in 1800, form 

 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. 



The work of DeSaussure, entitled " Recherches sur 

 la Vegetation," published in 1804, was the first sys- 

 tematic work showing the sources of the compounds 

 stored up in plant bodies. He demonstrated, quanti- 

 tatively, 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 main- 

 tained 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 believed that 

 the nitrogen of the soil was the main source of the 

 nitrogen found in plants. These views have since 



