86 ^ SCIENCE PRIMERS. ! [8 vi. 



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 tlie 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 v/ould 

 be bright scarlet, and that in the vein dark purple. 

 Hence we call the bright scarlet blood which is found 



