16 DISCOVERY OF PHOTOSYNTHESIS CHAP. 2 



matter to be of vegetable nature. How close he was, at this point, to the 

 final discovery of photosj^nthesis ! However, he let himself be deceived 

 by microscopic observations which revealed no organic forms in the green 

 matter, and bj^ the formation of this matter in closed vessels, and decided 

 that it was a thing "sui generis," of mineral rather than organic character. 

 Subsequent observations diverted him even further away from the right 

 track — he noticed that water decanted from the green matter also evolved 

 "purified air" upon shaking, and that even pure pump water, not visibly 

 contaminated with green matter, produced "purified air" upon prolonged 

 standing in sunlight. The picture thus became more and more confused, 

 as Priestley acknowledged in the following sentences: "It will probably 

 be imagined that the result of the experiments recited in this section 

 throws some uncertainty on the result of those from which I have con- 

 cluded that air is ameliorated by the vegetation of plants, and especially 

 as the water by which they were confined was exposed to the open air and 

 the sun in the garden. To this I can only say that I have represented 

 the naked facts, as I have observed them ; and having not great attach- 

 ment to any particular hypothesis, I am very willing that my reader 

 should draw his own conclusions for himself." (However, he added some 

 arguments which made him believe that the previously observed plant 

 effects were genuine and not due to the illumination of water.) 



Obviously, Priestley's careful attention to factual details and his re- 

 fusal to attach too much importance to hypotheses — in short, his purely 

 experimental approach — preve ted, at that time, his complete realization 

 of the true nature of photosynthesis. His doubts were cleared away two 

 years later, when he published the second volume of Observations and 

 Experiments in Natural Philosophy. By then, the green matter was 

 definitely identified as vegetable in nature. (It is interesting to reflect 

 that the same unicellular green algae, which have recently become the fa- 

 vorite subjects of photosynthetic study, served in the discovery of this phe- 

 nomenon over a century and a half ago.) The action of water decanted 

 from the green algal deposits, which baffled Priestley in 1779, was ex- 

 plained in 1781 as an effect of supersaturation with ox\^gen; the formation 

 of green deposit in closed vessels was attributed to imperfect closure and 

 contamination of water with "seeds" before corking. Thus, the picture 

 of oxygen being formed by the cooperation of green vegetable matter and 

 sunlight, emerged clearly from the temporary confusion. 



These results were obtained and published by Priestley in 1781; and 

 in the meantime, a development had taken place, which Priestley himself 

 had described, almost prophetically, four years earlier, when he wrote 

 (in the preface to the first volume of Different Kinds of Ai7-) : 



"I do not think it at all degrading to the business of experimental philosophy to 

 compare it, as I often do, to the diversion of hunting, where it sometimes happens that 



