To pursue this a little further, let us see what will happen with
lime-water. Here is a globe which contains a little lime-water, and it is
so arranged as regards the pipes, as to give access to the air within, so
that we can ascertain the effect of respired or unrespired air upon it. Of
course, I can either draw in air (through A), and so make the air that
feeds my lungs go through the lime-water, or I can force the air out of my
lungs through the tube (B), which goes to the bottom, and so shew its
effect upon the lime-water.

[Illustration: Fig. 34.]

You will observe that, however long I draw the external air into the
lime-water, and then through it to my lungs, I shall produce no effect
upon the water--it will not make the lime-water turbid; but if I throw the
air _from_ my lungs through the lime-water, several times in succession,
you see how white and milky the water is getting, shewing the effect which
expired air has had upon it; and now you begin to know that the atmosphere
which we have spoiled by respiration is spoiled by carbonic acid, for you
see it here in contact with the lime-water.

I have here two bottles, one containing lime-water and the other common
water, and tubes which pass into the bottles and connect them. The
apparatus is very rough, but it is useful notwithstanding.

[Illustration: Fig. 35.]

If I take these two bottles, inhaling here and exhaling there, the
arrangement of the tubes will prevent the air going backwards. The air
coming in will go to my mouth and lungs, and in going out, will pass
through the lime-water, so that I can go on breathing and making an