I have here [pointing to a globe of gold-fish] respire by the
oxygen which is dissolved from the air by the water, and form carbonic
acid; and they all move about to produce the one great work of making the
animal and vegetable kingdoms subservient to each other. And all the
plants growing upon the surface of the earth, like that which I have
brought here to serve as an illustration, absorb carbon. These leaves are
taking up their carbon from the atmosphere, to which we have given it in
the form of carbonic acid, and they are growing and prospering. Give them
a pure air like ours, and they could not live in it; give them carbon with
other matters, and they live and rejoice. This piece of wood gets all its
carbon, as the trees and plants get theirs, from the atmosphere, which, as
we have seen, carries away what is bad for us and at the same time good
for them,--what is disease to the one being health to the other. So are we
made dependent, not merely upon our fellow-creatures, but upon our
fellow-existers, all Nature being tied together by the laws that make one
part conduce to the good of another.

There is another little point which I must mention before we draw to a
close--a point which concerns the whole of these operations, and most
curious and beautiful it is to see it clustering upon and associated with
the bodies that concern us--oxygen, hydrogen, and carbon, in different
states of their existence. I shewed you just now some powdered lead, which
I set burning[18]; and you saw that the moment the fuel was brought to the
air, it acted, even before it got out of the bottle--the moment the air
crept in, it acted. Now, there is a case of chemical affinity by which all
our operations proceed. When we breathe, the same operation is going on
within us. When we burn a candle, the attraction of the different parts
one to the other is going on. Here it is going on in this case of the
lead; and it is a beautiful instance of chemical affinity. If the products
of combustion rose off from the surface, the lead would take fire, and go
on burning to the end; but you remember that we have this difference
between charcoal and lead--that, while the lead can start into action at
once, if there be access of air to it, the carbon will remain days, weeks,
months, or years. The manuscripts of Herculaneum were written with
carbonaceous ink, and there they have been for 1,800 years or more, not
having been at all changed by the atmosphere,