120 PROCEEDINGS OF SECTION B. 
the air had the most direct bearing, we are very liable to 
become too much involved in other sciences. 
The part played by the oxygen of the air in combustion 
and calcination—production of metallic oxides—led off into 
fertile fields of further chemical discovery. 
The geologist found in its conclusions a new agent for the 
breaking up and reconstructing of the earth, and evolved 
the branch science of chemical geology. There were, how- 
ever, especially two great branches of inquiry that were laid 
conspicuously and temptingly open for mvestigation. and 
which soon attracted the services of able men; these were 
the relations between atmospheric air and the lives of the 
plant and the animal. 
RELATION TO VEGETABLE LIFE. 
When we find Van Helmont, over 300 years ago, growing 
a willow tree 164 lbs. in weight from water alone, and find- 
img that the earth im which it grew had not appreciably 
altered in weight, we must not be hard upon him if we find 
him adopting fantastic explanations; for it doubtless is a 
very puzzling result. Stephen Hales took up chemistry to 
explain his botany, and, in 1727, he published the-results of 
ingenious experiments, demonstrating that air was inspired 
by plants, from which he tried to recover it. “‘ The air is 
very instrumental in the production and growth of animals 
and vegetables,” he writes. 
Priestley studied very carefully the effect of plants on the 
surrounding air, and came very close to a complete explana- 
tion of the process. He seems, indeed. to have reasoned 
out and believed what Ife did not succeed in completely de- 
monstrating. He saw clearly that animal respiration was 
closely related to combustion; and he set out on the quest 
for that natural provision which he was convinced must exist 
for restoring health to the atmosphere vitiated by the varied 
processes of animal respiration, combustion, and organic de- 
cay. He gave a clear demonstration that plants “‘ reverse 
the effect of breathing, and tend to keep the atmosphere 
sweet and wholesome,” and he recognised the great signific- 
ance of this action. Ingenhousz showed that this action was 
due to the green parts of plants, under the influence of suffi- 
ciently imtense light, and that the roots, always, and the 
green parts, m darkness, have an action similar to that of 
animals. 
It was, however, reserved for the younger Saussure to 
present a complete quantitative demonstration of these 
facts—that the green parts of the plant, under the action of 
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