10. Tobacco injures the blood. 11. Tobacco weakens the heart and makes the pulse irregular. 12. The use of strong tea and coffee causes palpitation of the heart. 13. A cold is caused by a disturbance of the circulation. A cold should never be neglected. 14. When an artery is wounded, the blood is bright red and flows in spurts. 15. When a vein is wounded, the blood is purple and flows in a steady stream. 16. To stop bleeding from an artery, press on the side of the wound towards the heart, or on both sides of the wound. 17. When a vein is wounded, press on the side away from the heart. CHAPTER XIII. WHY AND HOW WE BREATHE. ~1. An Experiment.~--Let us perform a little experiment. We must have a small bit of candle, a fruit jar, or a bottle with a large mouth, and a piece of wire about a foot long. Let us notice carefully what we are about to do and what happens. ~2.~ We will fasten the candle to the end of the wire. Now we will light it, and next we will let it down to the bottom of the jar. Now place the cover on the top of the jar and wait the results. Soon the candle burns dimly and in a little time the light goes out altogether. ~3.~ What do you think is the reason that the candle will not burn when shut up in a bottle? A candle uses air when it burns. If shut up in a small, tight place, it soon uses up so much air that it can burn no longer. Try the experiment again, and when the candle begins to burn dimly, take it out quickly. We see that at once the light burns bright again. ~4.~ Suppose we shut the stove draught tight, what is the result? The fire will burn low, and after a time it will probably go out. Why is this? Evidently the stove needs air to make the wood or coal burn, just as the candle needs air to make it burn. ~5. Animals Die without Air.~--If you should shut up a mouse or any other small animal in a fruit-jar, its life would go out just as the light of the candle went out. The little animal would die in a short time. A child shut up in a close place would die from the same cause in a very little time. In fact, many children are dying every day for want of a sufficient supply of pure air. ~6. Oxygen.~--The reason why animals need air, and why the fire will not burn without it, is that the air contains _oxygen_, and it is the oxygen of the air which burns the wood or coal and produces heat. So it is the oxygen that burns in our bodies and keeps us warm. ~7.~ When wood and coal are burned, heat is produced; but some parts of the fuel are not made into heat. While the fire burns, smoke escapes through the pipe or chimney; but a part of the fuel remains in the stove in the form of ashes. Smoke and ashes are the waste parts of the fuel.