264 OUR WOODLAND TREES. 
known as carbonic acid gas, reduce it to its simple 
and healthful elements of carbon and oxygen, 
assimilate the carbon by making it contribute to 
their substance, and hence to their life, vigour, 
and beauty, and give back pure oxygen—our vital 
air—for the healthfulness and pleasure of man- 
kind. How beautiful, indeed, is this function, 
whereby a deleterious gas is turned to the double 
advantage of human beings by the operations of 
Trees! We not only benefit by what is returned 
to us, but by what is retained to aid the Tree 
growth; for, as the carbon assimilated by the 
Tree is made to contribute to the perfection of its 
most beautiful, most useful, and most enduring 
qualities —to the charm of its graceful foliage, to 
the clustering profusion of its fruit, and to the 
solidity and stability of its timber, so, for the 
same reason it ministers to the further enjoyment 
of man by providing him with food in health, with 
medicine in sickness, with shelter to temper the 
heat of the summer sun, or the icy chill of the 
wintry wind, and with that inexpressible sense of 
pleasure conveyed to the mind through the eye 
by the presence of noble stem and _ spread- 
