But it has a power of combining with certain other things and forming substances which, because they are combinations of carbon, are called _carbonates_.

The commonest substance with which it will do this is lime.

If you take a glass or a bottle two-thirds full of lime water, and breathe into it through a glass tube or straw, you will see in a very few minutes that it is becoming milky or cloudy from the formation of visible carbonate of lime, which, when you get enough of it, makes ordinary limestone.

So, although you cannot see, or smell, this carbon "smoke" in your breath, you can readily prove that it is present.
[Illustration: "IMPROVING THEIR WIND"] How and Why our Breathing Varies.

When you run or wrestle, you breathe faster in order to draw more air into the lungs.