PREFACE 



Photosynthesis is the name now generally attached to 

 one of the fundamental cosmic processes, the one that 

 underlies the great primitive industry of Agriculture : it 

 is therefore a process which should be completely under- 

 stood. An idea of the present state of knowledge con- 

 cerning it will, perhaps, be most satisfactorily conveyed 

 by means of a brief historical sketch of the progress of its 

 discovery. As has been so often the case in scientific 

 research, the starting-point was an accidental observation. 

 In the account of his investigations on the air (1772), 

 Priestley remarks : ' I have been so happy as by accident 

 to have hit upon a method of restoring air which has been 

 injured by the burning of candles, and to have discovered 

 at least one of the restoratives which nature employs for 

 this purpose — it is Vegetation. The restoration of vitiated 

 air, I conjecture, is effected by plants imbibing the phlo- 

 gistic matter with which it is overloaded by the burning 

 of inflammable bodies.' Whilst Priestley was satisfied that 

 the significance of the process was the purification of the 

 air for animal respiration, his friend Percival reached the 

 further conclusion that ' fixed air is far from being destruc- 

 tive to vegetation, it is the proper pabulum of vegetables, 

 making them to flourish much more than they could do 

 in other circumstances,' which put the matter on the 

 right basis. 



Research on the subject was continued by Ingenhousz 

 and by Senebier, with considerable success. Ingenhousz 

 established the fact that the purifying action of plants on 



