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oxygen during the day. De Saussure stated that when a plant was 
confined in a definite volume of atmospheric air, that air was unaltered 
in composition after a definite time. Le Mavul and Descaisne, as 
translated by Hooker, asserted that plants, by day or night, poison the 
atmosphere, not only by the exhalation of carbonic acid, but also by 
liberating carburetted hydrogen, the product of the volatile oils to 
which the perfume of the corolla was due. 
Add to these conflicting testimonies that the last named authors 
accredited plants with the power of decomposing water, retaining the 
hydrogen, and liberating the oxygen, that Morot, Lory, and others 
said that the white parts of plants, or whole plants like the Orobanche 
having no green parts, exhaled carbonic acid always and only, that 
Ville and Gratiolet affirmed that all plants decomposed ammonia and 
exhaled nitrogen, and that the single species called Chenopodium 
vulvaria claimed the property of exhaling ammonia, and we had a 
fair idea of the chaos into which the enquirer into the subject must 
necessarily plunge. He could not, of course, hope to throw light on 
a subject confessedly so obscure, or to decide where such doctors 
disagree, but he thought that he might usefully occupy himself in 
weeding the garden which he was not competent to cultivate, by 
arranging the various statements, rejecting opinions which had been 
disapproved, tabulating such as were established, and shewing on 
what arguments questions yet undecided rested, and he might state, 
in justice to himself, that he had consulted most of the authorities, 
and repeated most of the experiments referred to. 
The statements requiring atttention were :—1. That leaves ex- 
posed to sunlight decompose carbonic acid and liberate the oxygen. 
2. That leaves, during the darkness, or always, convert oxygen into 
carbonic acid, which they respire. 3. That plants respire nitrogen. 
4. That Chenopodium vulvaria respires ammonia. 
The first position, that plants decompose carbonic acid under 
the influence of sunlight, was, he submitted, fully proved. A 
potted plant placed in a jar of carbonic acid would rapidly con- 
vert it to oxygen, if exposed to the noon-day sun. A sprig of mint 
placed in a bell glass containing water charged with carbonic acid, 
would, when exposed to sunlight, discharge bubbles of pure oxygen. 
The confervee of fresh water, and the algze of salt water, manifested 
the same property, as every one who had an aquarium knew. It 
might be also taken as proved that this operation ceased in the dark, 
and was confined to the green parts of plants. The rhizome of a 
potato discharged no oxygen; the yellow monotropa or brown /ucius 
