THE ORIGIN OF ORGANIC MATTER l5 



of our knowledge of photosynthesis. Priestley had noticed that plants con- 

 fined in an atmosphere rich in fixed air (carbon dioxide) produced in the 

 course of several days large quantities of dephlogisticated air (oxygen). 

 Priestley explained the i)henomenon as caused by the growth of the plants. 

 About the same time Scheele, working in Sweden, occupied himself 

 with the same subject. He arrived at results which were quite the opposite 

 to those of Priestley. While Priestley's plants improved the air with 

 oxygen, Scheele's plants produced carbon dioxide. As we now know, 

 the cause of this contradiction lay in the fact that neither Priestley nor 

 Scheele realized clearly under what external conditions the plant emitted 

 carbon dioxide and when it emitted oxygen. Priestley industriously re- 

 peated his investigations but became confused through the irregular out- 

 come of his experiments. 



In 1768 Jean Ingen-Housz, a Dutch physician, who had been studying 

 vaccination against small-pox in London, was called to Vienna to combat 

 this disease which was raging in that city and had claimed many victims 

 from the royal family. Ingen-Housz had wide scientific interests and 

 knowledge. It was through reading the address of Pringle on Priestley's 

 discoveries, which has already been referred to, that Ingen-Housz became 

 filled with the desire to repeat the experiments on the production of oxygen 

 by plants. 



Through his successes with vaccination he soon won the favor of the 

 Empress Maria Theresa and was finally given an annuity from the state 

 which permitted him to follow his own scientific investigations. Ingen- 

 Housz was at first primarily interested in the influence of foul and pure 

 air on the health of man. The discoveries of Priestley, already referred 

 to, served as a great stimulus to his studies and were the beginning of his 

 fundamental discoveries. Thus, in the preface to his "Experiments upon 

 Vegetables" (1779) he wrote: "The discovery of Dr. Priestley that plants 

 thrive better in foul air than in common and in dephlogisticated air, and 

 that plants have a power of correcting bad air, has thrown a new and im- 

 portant light upon the arrangements of this world. It shews . . -that 

 the air, spoiled and rendered noxious to animals by their breathing in it, 

 serves to plants as a kind of nourishment." 



Ingen-Housz was much more fortunate in his experimentation than 

 either Priestley or Scheele. He soon saw that the mere growth of a plant 

 had nothing to do with the purification of the air. His experiments are 

 masterpieces of manipulation and self-criticism. It should be remembered 

 that this was before Lavoisier had established the nature of combustion. 

 Step by step Ingen-Housz approached the correct interpretation of the 

 phenomenon. The plants were able to purify bad air in a few hours when 

 subjected to sunlight. He discovered that they absorb the air and exhale 

 oxygen ; that this action is the more active the brighter the sunlight, that 

 in the shade the activity is less while in deeper shade and at night, plants, 

 far from purifying the air, contaminate it as animals do; that only leaves 

 and petioles can accomplish this, and that mature leaves give ofif more 



