You have learnt from your Chemistry Primer (Art. 2, p. 2) what happens when a candle is placed in a closed jar of pure air. The oxygen gets less, carbonic acid comes in its place, and after a while the candle goes out for want of oxygen to carry on that oxidation which is the essence of burning. You also know that exactly the same thing would happen if you were (only you need not do it) to put a bird or a mouse in the jar instead of a candle. The oxygen would go, carbonic acid would come, and the little flame of life in the mouse would flicker and go out, and after a while its body would be cold. But suppose you were to put a fish or a snail in a jar of pure fresh water, and cork the jar tight. There seems at first sight to be no air in the jar. But there is. If you were to take that fresh water, and put it under an air-pump, you could pump bubbles of air out of it; and if you were to examine these bubbles you would find them to contain oxygen and nitrogen, with very little carbonic acid. =The water contains dissolved air.= After the fish or the snail had been some time in the jar, you would see its flame of life flicker and die out, just like that of the bird in air; and if you then pumped the air out of the water you would find that the oxygen was nearly gone and that carbonic acid had come in its place. You see, then, that air can be breathed, as we call it, even when it is dissolved in water. Now to return to our muscle. When you were watching the circulation in the frog’s foot, you could tell the artery from the vein, because in the artery the blood was flowing _to_ the capillaries, and in the vein _from_ them. Both artery and vein were rather red, and of about the same tint of colour. But if you could see in your own body a large artery going to your biceps muscle, and a large vein coming away from it, you would be struck at once with the difference of colour between them. The artery would look bright scarlet, the vein a dark purple; and if you were to prick both, the blood would gush from the artery in a bright scarlet jet, and bubble from the vein in a dark purple stream. And wherever you found an artery and a vein (with a great exception of which I shall have to speak directly), the blood in the artery would be bright scarlet, and that in the vein dark purple. Hence we call the bright scarlet blood which is found in the arteries =arterial blood=, and the dark purple blood which is found in the veins =venous blood=.