THE FOOD OF GREEN PLANTS 79 



A Dutch physician, named John Ingenhousz, who was 

 then living- in England, read Priestley's narrative and 

 began to investigate on his own account. Without 

 detailing his numerous experiments, we may give his 

 own clear summary (condensed). " I observed," Ingen- 

 housz says, " that plants have a faculty to correct bad 

 air in a few hours ; that this wonderful operation is due 

 to the light of the sun ; that it is more or less brisk 

 according 1 to the brightness of the light ; that only the 

 green parts of the plant can effect the change ; that 

 leaves pour out the greatest quantity of oxygen from 

 their under surfaces ; that the sun by itself has no 

 power to change the composition of air." It will be 

 seen that Priestley started the inquiry, devised and 

 executed the most necessary experiments, and got 

 excellent results. Then he lost his way, and bewil- 

 dered by conflicting observations, which he was too 

 impatient to reconcile, published a barren and mis- 

 leading conclusion. Nothing was left for him but to 

 acknowledge that Ingenhousz had cleared up all his 

 perplexities. 



Nicholas Theodore de Saussure, son of the Alpine 

 explorer, showed in 1804 that when carbon is separated 

 from the carbonic acid of the air by green plants, the 

 elements of water are also assimilated, a result which 

 owes its importance to the fact that starch is a combina- 

 tion of carbon with the elements of water. Saussure 

 also proved that salts derived from the soil are essential 

 ingredients of plant-food, and that green plants are 

 unable to fix the free nitrogen of the air ; all the nitrogen 

 \vhich they require is obtained from the ground. 



We are unable to follow the history further. Though 

 the main facts were established as early as the begin- 

 ning of the nineteenth century, experimental results of 



