18 
PLANTS. 
pliers, to bear a great proportion to the moisture 
imbibed. We learn from the experiments of Mr. 
Woodward, that a sprig of mint in seventy-seven 
days absorbed 2558 grains of water, and yet its 
weight was only increased fifteen grains ; therefore 
it must have perspired 2543 grains. Several other ex- 
periments were tried with similar results ; all tend- 
ing to prove the great evaporation which is conti- 
nually taking place from the surface of plants. It 
must be remarked that light and heat greatly pro- 
mote this perspiration ; for in the night a much 
less quantity is carried off, and little or none during 
a frost, or while it rains. 
Absorbing Poiver of Plants. 
A very singular part of the oeconomy of vegetable 
substances (which proves, by the way, how care- 
ful Nature has been to make this part of the crea- 
tion essentially serviceable to our health,) is their 
ability to absorb the bad air from the atmosphere, 
and at the same time to return what is fit for respi- 
ration in the room of it. We are indebted for this 
singular discovery to the indefatigable Dr. Priestley, 
who, in the year 1771 , made a sprig of mint vege- 
tate for ten days, in a quantity of air which had been 
previously vitiated and rendered unfit for respiration : 
in consequence of this vegetation, however, the 
bad air was so far improved as to suffer a candle to 
burn in it, and to be very nearly restored to its ori- 
ginal purity. This pure air, or oxygen gas, is emit- 
