200 
R. J . Harvey Gibson. 
amount of air given off by leaves in water charged with different 
proportions of carbon dioxide, while the third part summarises the 
results obtained in the preceding chapters. 
A perusal of Senebier’s works forces one to the conclusion 
that, though a good observer and a laborious experimenter, he is 
far from being a clear thinker. His style is frothy and turgid, and 
he repeats himself again and again, and in this respect offers a 
marked contrast to Ingenhousz. The latter sets before himself a 
definite problem and contrives an experiment to prove his point. 
Senebier, on the other hand, piles experiment on experiment, but 
leaves his reader to draw the conclusion, for there is little or 
nothing of a general thesis running through the work to string the 
experiments together. He repeated Ingenhousz’s work on the 
relation of sunlight to the exhalation of gas from green parts and 
held that the leaves were the responsible agents for this phenome¬ 
non, while non-green parts gave off C0 2 only. He shewed that it 
was light and not heat that induced the exhalation of oxygen and 
that this took place only in presence of C0 2 . It is generally said 
that Senebier discovered that the oxygen resulted from the 
decomposition of C0 2 but I have been unable to discover any 
evidence to support this view. On the contrary, he says that the 
plant takes the phlogiston out of the C0 2 and gives off oxygen as 
an excrement. He thinks that C0 2 is thus changed into oxygen 
and that the acid character of the C0 2 acts as a stimulant on the 
leaves to bring about this change. He attempted indeed to induce 
the exhalation of oxygen by substituting hydrochloric, sulphuric and 
nitric acid for carbonic acid, but naturally without the desired 
result. He also believed that the C0 2 entered by the root. 
Holding such views as these it is scarcely to be expected that 
Senebier could have had any conception that C0 2 was decomposed 
into carbon and oxygen when leaves were exposed to sunlight. It 
is true, he claimed in his “ Physiologie Vegdtale,” published in 1800, 
that he had an idea of the decomposition of C0 2 in 1788, but 
Ingenhousz had also a suspicion of the same phenomenon in 1779, 
and at the date of the publication of the general treatise in which 
he made this claim, Ingenhousz’s “ Nutrition of Plants” had been 
before the scientific public for four years, and in it the thesis is 
clearly enunciated, viz., that carbon dioxide is decomposed in 
sunlight, that the carbon is retained by the plant and that the 
oxygen is exhaled. Hansen’s estimate of Senebier seems most 
nearly to approach the truth :—“ Dtmnach kann Senebier weder 
