"So," said Harry, "strictly pure water would not be quite so great a blessing to us as you might think. Of course, you know, uncle, I don't mean to say that there is any advantage in the impurity of such water as the Thames, except when used for the purpose of fertilizing the earth. I am speaking of water so pure as to contain no air. Water of such severe purity would be very unmanageable stuff. No fishes could live in it, for one thing. I have already given you one good reason why it would be unsuitable to our kettle; and another is, that it would not be good to drink. Then water, as we find it in the world, has a very useful and accommodating disposition to find its own level. Pump all the air out of water, however, and it loses this obliging character in a great measure. Suppose I take a bent glass-tube, and fill one arm of it with airless water. Then I turn the tube mouth upward, and if the water were common water, it would instantly run from one arm into the other, and stand at the same level in both. But if the water has been exhausted of its air, it remains, most of it, in the one arm, and won't run till I give the table a smart rap, and shake it. So, but for the air contained in water, we could not make the water run up and down hill as we do. If water were deprived of air, London would be almost deprived of water." "And water," observed Mr. Bagges, "would be robbed of a very valuable property." "Good again, uncle. Now, if we could see through the kettle, we should be able to observe the water boiling in it which is a curious sight when looked into. To examine water boiling, we must boil the water in a glass vessel--a long tube is the best--heated with a spirit lamp. Then first you see the water in motion, and the air-bubbles being driven off by the heat. As the water gets hotter, other bubbles appear, rising from the bottom of the tube. They go up for a little way, and then they shrink, and by the time they get to the top of the water, you can hardly distinguish them. These are bubbles of steam, and they get smaller as they rise, because at first the water is colder above than below, in proportion to the distance from the flame, and the cold gradually condenses the bubbles. But when the water gets thoroughly hot, the bubbles grow larger and rise quicker, and go of the same size right up to the top of the water, and there escape--if you choose to let them. And steam was allowed to escape so for many, many ages, wasn't it, uncle, before it was set to work to spin cotton for the world, and take us to America within a fortnight, and whirl us over the ground as the crow flies, and almost at a crow's pace?" "For all which," remarked Mr. Bagges, "we have principally to thank what's his name."