88 LIFE OF A TREE. 
mind the enormous mass of impurity daily thrown 
into the atmosphere from one locality alone, and 
to stimulate the reader to follow us with interest 
as we proceed to inform him how, by a wise and 
admirable regulation, it is all removed from the 
air again; if indeed he has not already antici- 
pated the solution to the difficulty. Let us 
bear in mind then, that all men, all animals 
and birds, and all combustion of fuel, are engaged 
in filling the air with carbonic-acid gas, which 
has been already said to be produced in breathing, 
and from burning coal and other fuel. The ten- 
dency of all this is to fill the air with poison,— 
for carbonic acid is poisonous,—and consequently 
to render it quite unfit for us to live in. And 
there is not the least doubt that in time this 
would actually take place, and the wide world 
would not own either a man, beast, bird or 
insect, all having perished by inhaling the deadly 
atmosphere. 
Now comes the beautiful discovery of the 
office of the vegetable world to our relief; for 
we find here a law at work which exactly 
neutralizes all the poison, and restores its lost 
purity to the atmosphere again. He who or- 
