820 The American Naturalist. [October, 
candle does it act in the living substance. The oxidations are 
not direct in living matter as in the candle, but the living ` 
matter first takes the oxygen and makes it an integral part of 
itself, as it does the carbon and nitrogen and other elements; 
and finally when energy is to be liberated, the oxidation 
occurs, and the carbon dioxide appears as a waste product. 
The oxygen that is breathed to-day, like the carbon or the 
nitrogen that is eaten, may be stored away and represent only 
= so much potential energy to be used at some future time in 
mental or physical action. 
So far only living animal substance has been discussed. If 
plants are considered what can be said of their relations to 
the air? The answer was given in part by Priestley (1771), 
who found that air which had been vitiated by animal respi- 
ration became pure and respirable again by the action of green 
plants. He thus discovered the harmonizing and mutual 
action of animals and plants upon the atmosphere; and there 
is no more beautiful harmony in nature. Animals use the 
oxygen of the air and give to it carbon dioxide, which soon . 
renders it unfit for respiration ; but the green plants take the 
carbon dioxide, retain the carbon as food, and-return the oxy- 
gen to the air as a waste product. This is as thoroughly 
established as any fact in plant physiology, and yet in his 
work Priestley had some which ke called “bad exper- — 
iments,” for instead of the plants giving out oxygen and 
purifying the air they sometimes gave off carbon dioxide, and 
_ thus rendered it more impure, after the manner of an animal. 
What investigator cannot sympathize with Priestley when he 
calls these “bad experiments?” They appeared so rudely to 
put discord into his discovered harmony of nature. But 
nature is png ie greater than man dreams. The “bad 
experiments ” were among the most fruitful in the history of 
scientific discovery. Ingenhausz (1787) followed them up, 
carefully observing all the conditions, and found that it was 
only in daylight that green plants gave out oxygen; in dark- 
ness or insufficient light they conducted themselves like ani- 
mals, taking up oxygen and giving out carbon dioxide. 
