TRANSACTIONS OF SECTION B. 667 
process as a purely chemical one, due to the direct action of light on the carbon 
dioxide dissolved in the water. 
But this was by no means Priestley’s final view, as shown by a further descrip- 
tion of his experiments on plants set forth in the new edition of his works published 
in 1790, where he clearly recognised the error into which he had been led.! 
Meanwhile the subject had been taken up by two other observers, Incen-Housz 
and Senebier, and in order to thoroughly understand the respective shares which 
these men took in advancing our knowledge of the assimilatory process, it is 
necessary to consult not only their books but also the numerous scattered memoirs 
which appeared at intervals between the years 1779 and 1800. 
To Ingen-Housz must unquestionably be awarded the merit of having experi- 
mentally demonstrated that the amelioration of the surrounding air by plants is 
not, as Priestley at first believed, due to vegetative action per se, but is dependent 
on the access of light of a sufficient degree of intensity, and, moreover, that the 
power is confined to the green parts of the plants. At the same time, whilst 
recognising, as Priestley had done before him, that the combined action of plants 
and light on the air was a dephlogisticating process, he did not know, until after 
its demonstration by Senebier, that the particular form of phlogisticated air which 
was essential to plants was ‘fixed air’ or carbon dioxide. In fact Ingen-Housz had 
but a slender knowledge of the chemistry of his day, so much so indeed that he 
constantly confuses ‘ phlogisticated air’ or nitrogen with ‘fixed air,’ and attributes 
the source of the evolved oxygen either to air imprisoned within the leaf, or, in 
the case of submerged plants, to a metamorphosis of the water itself. I must, 
however, recall the fact that Ingen-Housz was the first to show that the green 
parts of plants in the dark, and the roots both in the light and in darkness, vitiate 
the air in the same way as animals do. On the strength of these experiments he 
is generally given credit for having first observed the true respiration of plants, but 
I cannot avoid the conclusion that, in the controversy which ensued on this point 
between Ingen-Housz and Senebier, the adverse criticisms of the latter were well 
founded. Whilst not denying that plants in the dark have some mepbitic influence 
on the air around them, Senebicr maintained that the greater part of the observed 
effect was due to a fermentative action set up in the large bulk of leaves which 
Ingen-Housz employed. Certainly some of the results appear to be largely in 
excess of those we should now expect to obtain from respiratory processes only.” 
Senebier’s work falls between the years 1782 and 1800. The fact that he was 
an early convert to the new ideas and generalisations of Lavoisier gives his views 
on plant nutrition far greater precision than those of Priestley and Ingen-Housz. 
His experiments, for the most part well devised, proved beyond all doubt that the 
oxygen disengaged from submerged and insolated plants could not be derived from 
air contained in the leaf parenchyma, but that it depended on the pre-existence of 
carbon dioxide, and that its evolution was strictly proportional to the amount of 
carbon dioxide which the water contained. 
‘ The view which was taken by Priestley’s ccntemporaries of his position with 
regard to the discovery of the fundamental facts is well exemplified by the following 
remarks taken from a paper published by Ingen-Housz in 1784 (Annales de Physique, 
xxiv. 44): ‘C’est 4 M. Priestley seul que nous devons la grande découverte que les 
végétaux possédent le pouvoir de corriger l’air mauvais, et d’améliorer l’air commun ; 
@est lui qui nous en a ouvertla porte. J’ai été assez constamment attaché a ce bean 
systéme, dans le temps que lui-méme, par trop peu de prédilection pour ses propres 
opinions, paroissoit chanceler.’ 
* It is by nomeans uncommon to find Ingen-Honsz put forward as the discoverer 
of the fixation of carbon by plants from carbon dioxide. This claim is generally 
based on certain statements made in his essay on the ‘Food of Plants and the 
Renovation of the Soil,’ published in 1796 as an appendix to the outlines 
of the fifteenth chapter of the Proposed General Report from the Board of 
Agriculture. All that is good and sound in this essay is taken from Senebier’s papers 
without any acknowledgment, but, in appropriating ideas which he evidently under- 
stands very imperfectly, he has built up a system of plant economy which is almost. 
unintelligible. é, 
