180 VEGETATION AND THE ATMOSPHERE. 
light exerts over the green portions of vegetables so strong an attraction that, 
if placed in the dark, they direct themselves towards the smallest openings 
which convey it to them. The second showed that when plunged in water, 
plants disengage in sunlight a large quantity of azr ; but there Bonnet stopped: 
he knew not what that air was, nor could he know it, since at that epoch the 
first principles of modern chemistry were known to none. 
Priestley, who was the rival and in some respects the predecessor of Lavoi- 
sier, was led by the results of his discoveries to study the action of plants on 
the atmosphere. He had just succeeded in isolating the remarkable gas which 
energetically supports the combustion of a lamp and the respiration of animals, 
and had for this reason called it vital air. He had ascertained, moreover, that 
small animals confined in this air or in atmospheric air soon changed its proper- 
ties to such an extent that the animals died and the flame was extinguished. 
_ It is true, Priestley did not know the real nature of oxygen, and through a blind 
feeling of rivalry refused to the end to adopt the theory of respiration just an- 
nounced by Lavoisier ; but he knew, nevertheless, how to deduce from his experi- 
ments a logical consequence which was of the greatest importance. Perceiving 
that these little animals vitiated the confined air by their exhalations, he com- 
prehended that all the individuals of the animal kingdom produce the same ‘ef- 
fect continually on the entire atmosphere, and that they must infallibly die, if 
there were not in the action of natural forces some inverse action constantly 
tending to restore to the air its purity, in proportion as this was destroyed by 
animal respiration. He proceeded to seek for this counterpoise, this regenera- 
tive action, and he found it in vegetables. 
He placed in the air confined under a glass bell an animal anda plant. The 
former corrupted the air and died; but atter the lapse of a certain time, Priest- 
ley discovered that the latter had restored to the air the vital property or the 
purity necessary to support life. This was one of the most important facts of 
the mechanism of our world. From this moment, it was known, though not 
yet in its details, that vegetables and animals execute antagonistic functions, 
these rendering the air untit for the support of life, those repairing the mischief. 
The Royal Society of London conferred on Priestley, in 1773, the Copley 
medal, and in presenting it, the president of that celebrated company thus 
characterized the discovery of Priestley: 
‘¢Plants do not grow in vain; each individual in the vegetable kingdom, from the oak of 
the forest to the grass of the field, is useful to the human race. All plants contribute to 
maintain our atmosphere in the degree of purity necessary to animal life. The forests, even, 
of the most remote countries contribute to our preservation, while deriving nourishment from 
the exhalations of our bodies which have become injurious to ourselves.” 
This glory of Priestley, however, was to be overclouded. After such success- 
fut labors, such grand and comprehensive views, such rewards and public honors, 
Priestley desired to repeat his former experiments, and obtained wholly oppo- 
site results; plants, instead of purifying the air, now appeared to him to pollute 
it. Surprised at this inexplicable contradiction between the past and the pres- 
ent, he multiplied and varied his experiments, and all that he could substan- 
tiate was, that vegetables possess the property at one time of purifying, at 
another of vitiating the air. he law which had won for him the Copley medal 
was therefore not general, and the consequences he had drawn from it not in- 
contestable. A refugee in America, after a life agitated by religious discussions, 
Priestley died in 1804, having made in chemistry brilliant discoveries which he 
did not comprehend, and in vegetable physiology contradictory experiments 
which he was not able to reconcile. 
Yet Priestley was deceived in nothing; plants do in fact alternately perform 
the two functions which he had assigned to them, and the only thing which he 
had not diseovered was the condition which determines, frequently the restora- 
tive, occasionally the deleterious action, a condition which Bonnet had caught 
