How steam is like air.
Steam is like air in three things. It is very thin; it is very elastic, or has a great deal of springiness; and you can not see it. Now perhaps you will say that this last is not true, and that we often see steam puffing out of a steam-engine or out of a tea-kettle; but this that we see is not really steam. It is not like the steam that is in the boiler of the engine or in the tea-kettle. It is a cloud of fog that the steam has turned into on coming out into the air. It is just like common fog, except that it is hot. Real steam you can not see as you see this.
Steam in boilers and tea-kettles.
Perhaps you will ask how I know that we can not see steam, as I can not look into a boiler or a tea-kettle. If we boil water in a glass vessel, we can see the steam if it can be seen; but we see nothing in the vessel over the water, and yet we know that there is a plenty of steam there, for the steam-fog is made in the air by the steam coming out at the mouth of the vessel.
How steam is made.
But we do not need this proof to show us that steam can not be seen. Look at the nose of a tea-kettle when the water is boiling in it quite briskly. Close to it, for half an inch or more, you can not see the steam-fog at all. What is the reason? There is a stream of steam coming out as fast as it can get out, but the air has not yet had a chance to change it into fog. It must spread out a little first. When it begins to spread out, the cool air makes the particles of steam form into companies, and it is a multitude of these companies that you see in the cloud of steam, as it is called, that comes from a steam-engine or from a tea-kettle. The air really changes the steam into water, for fog, as I have told you in Chapter XIX., is water in companies that are too small to make drops.
Simmering.
See, now, how steam is made out of the water in a tea-kettle. The fire heats the water that is nearest to it in the kettle. This rises, and more water comes to take its place and be heated, and so the water keeps circulating up and down, the warmer going up and the cooler going down. After a while, when the water all gets to be very hot, you hear a simmering noise. Now the steam begins to be made. The sound is made by little bubbles of steam which are formed at the bottom of the kettle. Soon larger bubbles of steam are made, because so much more of the water becomes hot enough to be readily made into steam; and the rising of these bubbles makes a great commotion, as you can see if the water be in an open pot. All this process of steam-making you can see if the water is boiled in a thin glass bottle, or flask, as it is called.
Force of steam.
There is a great deal of force in steam. It is steam that works the locomotive, and moves along the great steamship in the water. Sometimes it shows its power in destruction, as when it bursts a boiler.