It was for this reason that in § 22 the weight of the same volume of water was said to be constant under the same conditions; and, of course, the same qualification must be borne in mind when we speak of the weight of a cubic inch of water being about 252 and a half grains. Its weight is in fact 252·45 grains only when Fahrenheit’s thermometer stands at 62°—but as this is the temperature of ordinary mild weather, and the expansion or contraction of water for a degree about this temperature amounts to less than 1
3000th of its volume, the weight of a cubic inch may for all practical purposes be taken as 252 and a half grains.
32. Increase of Heat at length causes Water to become Steam.
Thus a change is effected in the properties of water by heating it ever so little. If it is more strongly heated a still greater change takes place. You know what happens when a saucepan containing water is put on the fire. The water gets hotter and hotter, then it begins to simmer, and finally, when it reaches 212°, it boils away into steam, which passes into the air and disappears. If the boiling is carried on long enough all the water vanishes. It looks at first as if the water had been destroyed by the heat. In reality, however, not a particle of water has been destroyed. It has merely changed its state. The heat has altered it from the state of liquid water into that of gaseous water, vapour or steam.
Try the same experiment with a tea-kettle instead of a saucepan, but only put a little water in the tea-kettle, and shut the lid well down. Then, as soon as the water begins to boil, the steam will shoot out of the spout in a jet; and this will go on as long as any water remains in the kettle.
The steam, as it comes out of the spout, is so hot that it will scald you if you put your finger in it. But you may satisfy yourself that it is very hot, without scalding your fingers, by holding a stick of sealing-wax in it. The wax will soften, just as if you held it before the fire. Moreover, if you look through the steam, just where it leaves the spout, you will see that it is quite transparent; it is only at some little distance from the spout that it loses its transparency, changes into a white opaque cloud, and rapidly vanishes in the air.
33. The taking away of Heat from Steam causes the steam to change into Hot Water.
Now take a cold spoon, or a cold plate, and hold it against the jet of steam, for a moment or two. When you take it away, you will find that it is quite wet, being covered with drops of warm water, and, moreover, the cold spoon, or plate, has become warm. And if you fit a long cold metal pipe to the nozzle of the tea-kettle, you will find that no steam at all issues from the end of the pipe, but only water, while the pipe becomes warmed.
Thus the heat passes from the fire into the saucepan, or kettle, and thence to the water which they contain; the water gets hotter and hotter, and, when it has taken in a certain quantity of heat, it becomes steam, or vapour of water. When the steam comes against the cold plate, or passes through the cold pipe, it gives up the heat it has taken in to the plate, or the metal of the pipe. They carry off the heat which kept the water in a condition of a vapour, and so it passes back into the condition of liquid.
Thus steam and water are two conditions of the same thing, water; they are effects of the quantity of heat which the water has taken in.