What the Breath Is. As you know, your "breath," as you call it,--that is to say, the used-up air which you blow out of your lungs,--is different in several ways from pure, or unused air. In the first place, it is likely to have a slight musky or mousy odor about it. You never like to breathe any one else's breath, or have any one breathe in your face. This dislike is due to certain gases, consisting of impurities from the blood, the cells of the lungs, the throat, the nose, and, if the mouth is open, the teeth. These are not only offensive and disagreeable to smell, but poisonous to breathe. Then your breath is much warmer than the rest of the air. In fact, on a very cold morning you may have tried to warm up your fingers by breathing on them; and you have also noticed that if a number of people are shut up in a room with doors and windows closed, it soon begins to feel hot as well as stuffy. This heat, of course, is given off from the blood in the lungs and in the walls of the throat and nose, as the air passes in and out again. When you stand at the window on a cold day, the glass just in front of your mouth clouds over, so that you can no longer see through it; and if you rub your finger across this cloud, it comes away wet. Evidently, the air is moister than it was when you breathed it in; this moisture also has been given off from the blood in the lungs. But what of the principal waste gas that the blood gives off in the lungs--the carbon "smoke," or carbon dioxid? Can you see any trace of this in the breath? No, you cannot, for the reason that this gas is like air, perfectly clear and transparent, and never turns to moisture at any ordinary temperature. 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"]