LECTURE IV 



THE PIONEER INVESTIGATORS IN PHOTOSYNTHESIS 



MORE important even than the discoveries of Koelreuter 

 and Sprengel in relation to pollination were those of 

 Ingen-Housz, who, with the single exception of Hales, 

 stands head and shoulders above all the plant physio- 

 logists of the eighteenth century. His work indeed lays 

 the foundation-stone of all our knowledge of plant nutri- 

 tion, and hence I must consider it in some detail. 



In the year 1754 the Scottish chemist Black isolated 

 and identified what he called " fixed air," rechristened 

 " carbonic acid " some years later by Lavoisier. Mayow, 

 in 1674, had already discovered a gas which he termed 

 " spiritus nitroaereus " (oxygen), but the importance of 

 his discovery was overlooked until, just one hundred 

 years later, in 1774, Priestley rediscovered it and named 

 it " dephlogisticated air," also renamed " oxygen " by 

 Lavoisier. The publication of Priestley's Experiments 

 and Observations on Different Kinds of Air marks an 

 epoch in the history both of chemistry and of vegetable 

 physiology. In this famous work the author speaks of 

 " the restoration of air, in which a candle has burnt out, 

 by vegetation," and puts forward the thesis that this 

 purification is effected by " plants imbibing the phlogistic 

 matter with which it is overloaded by the burning of 

 inflammable bodies," and adds that he " generally found 

 five or six days were sufficient to restore the air when the 

 plant was in its vigour." Book IX. deals with " Observa- 



62 



