ee 
VEGETATION AND THE ATMOSPHERE. 181 
_ 
sight of, and which Ingenhousz was soon to place in open day. Ingenhousz was 
born at Breda, in 1730; he was a physician, and came to England to observe 
the process of inoculation for the small-pox, which was then beginning to be 
practiced. It was during this visit that he became acquainted with the labors 
of Priestley, and resolved to explain their contradictions; this he succeeded in 
doing in 1779, and he has himself recorded his discovery in these words: 
“*Searcely had I entered upon these researches, when the most interesting views presented 
themselves. I observed that plants not only possess the faculty of correcting impure air in 
six or more days, as the experiments of M. Priestiey seem to indicate; but that they accom- 
plish this important office, in the most complete manner, in the course of a few hours; that 
this surprising operation is by nu means owing to vegetation, but to the influence of the 
light of the sun upon the plants; that it commences only after the sun has for some time 
risen above the horizon, and that it is completely suspended during the darkness of the 
night; that plants shaded by tall buildings, or by other plants, do not perform this function— 
that is, they do not purify the air, but, on the contrary, exhale a deleterious air, and diffuse a 
real poison through the atmosphere which surrounds us; that the production of healthy air 
grows languid towards the close of day, and entirely ceases at sunset; that all plants cor- 
rupt the ambient atmosphere during the night; that all parts of the plant are not engaged in 
purifying the air, but only the leaves and green branches; that bitter, ill-smelling, and even 
poisonous plants perform this office equally with those which difluse the sweetest scents and 
are most salutary, &c.”’ * 
Ingenhousz had thus succeeded in discovering the force which occasions the 
respiration of plants. That force which had not before been suspected is from 
the sun, is ight. It diffuses itself over the leaves, which absorb it, and fulfils 
the vast work of regenerating the atmosphere. Thus far the most important, 
as also the most ditlcult part of the task was achieved; but there remained 
yet quite as much to be done. The sciences resemble the sieve of the Danaides ; 
each one tries to fill it; no one succeeds, because every discovery discloses a 
new horizon and presents a more remote goal which is never attained. After 
Ingenhousz, it still remained to be asked in what consisted that alteration of the 
air which animals occasion, and the remedy which vegetables supply. It be- 
longed to chemistry to answer, and Lavoisier, though not specially operating 
with that yiew, furnished the solution of this new problem. He furnished it on 
that day when he showed that animals absorb oxygen, burn slowly the organic 
materials with which they are nourished and return by expiration a quantity 
of carbonic acid containing all the carbon which they have consumed. “The 
' vitiated or corrupted air, as Priestley and Ingenhousz called it, was consequently 
air deprived of oxygen and charged with carbonic acid, and, since plants purify 
it, this clearly indicates that they decompose that carbonic acid, retaining the 
carbon and restoring the oxygen to the atmosphere. 
Considering the then existing state of chemistry, it might be thought that 
every one would have divined and proclaimed this explanation. It was not so, 
however, and new experiments were needed to discover it. It was a Genevese 
who had commenced this long train of deductions, and it was another Genevese 
who had the honor of completing it. His name was Sennebier; he had been 
the friend of Charles Bonnet, and to his example owed pursuit of the sciences 
as well as the councils which determined him to the study of the relations of 
plants and the air. He ascertained that vegetables placed in boiled water dis- 
engage no gas in the sun, but that they develop oxygen in abundance when 
the water has been previously charged with carbonic acid. He thence con- 
cluded that this gas is necessary to the respiration of plants, that it is decom- 
posed by them, and thus had the honor of announcing the law already prepared 
and discovered by his predecessors. The question might have now been justly 
considered as solved; but during these labors, which had occupied more than half 
a century, many errors had become mingled with the truths obtained, and con- 
tradictory assertions threw doubt upon different points of detail. A review of 
* Experiences sur les Vegetaux, par. 1, Ingenhousz, 1780, 
