37. Gases and Vapours.
Air is as much a gas in the coldest winter as it is in the hottest summer. But air can be liquefied by exposing it to a very low temperature, while, at the same time, it is subjected to an extremely great pressure. Thus, the difference between gases like air, which are condensed with extreme difficulty, and gases like steam, which are condensed easily, is only one of degree. Nevertheless there is a certain convenience in distinguishing those gases, which, like steam, are easily condensed as vapours. In what we ordinarily call steam, all the water of which it is composed remains gaseous only at and above the temperature of boiling water (212° Fahrenheit). Cooled ever so little below this point, most of it becomes condensed into hot liquid water. However, it must be recollected that though that particular form of gaseous water which we call steam exists only at and above the temperature of boiling water, yet water is capable of existing in the gaseous state down to the freezing-point.
Suppose that when our boiling flask contained nothing but water and steam, the mouth were stopped and the lamp removed. Then, so long as the temperature of the whole remained at that of boiling water, every cubic inch of steam above the water in the flask would weigh about ⅐th of a grain, since 100 cubic inches weigh about 15 grains. Suppose the capacity of the flask, exclusively of the fluid water in it, to be 100 cubic inches. Then, to begin with, the gaseous water which it contains will weigh 15 grains. If the flask is now allowed to cool, more and more of the gaseous water condenses into the fluid state; but, even down to the freezing-point, some water will remain in the gaseous state and will fill that part of the flask which is unoccupied by the fluid water. At blood-heat (98°) the gaseous water weighs only about a grain, though it still occupies 100 cubic inches; at the ordinary temperature of the air it weighs not more than ⅓rd of a grain; while, at the freezing-point, its weight is only ⅛th of a grain. But inasmuch as there is less and less actual weight of water in the same volume of gaseous water as the temperature falls, it follows that the density, or specific gravity, of the gaseous water must be less the lower the temperature. Moreover, while, at the boiling-point, gaseous water or steam resists compression with exactly the same force as air does, the lower the temperature the more easily compressible is the gaseous water.
Suppose an elastic bag were to be tied on to the nozzle of a kettle full of boiling water. If the bag were kept as hot as the boiling water it would become fully distended, and maintain its shape in spite of the pressure of the air upon all sides of it. If the bag were taken away it would retain its shape so long as it was kept as hot as boiling water; but, if it were allowed to cool, it would gradually become flattened by the outside air squeezing up the less and less resisting gaseous water of the lower temperatures. Hence, when the stopped flask has been allowed to cool, the air rushes in with great violence if it is opened.
38. The Evaporation of Water at ordinary Temperatures.
If some water is poured into a saucer and is allowed to stand even in a cool room or in the open air, you know that it sooner or later disappears. Wet clothes hung on a line soon dry—that is to say, the water clinging to them disappears or evaporates. The disappearance of the water under these circumstances results from the property just mentioned. In fact, it becomes gaseous water of the density appropriate to the temperature, and as such mixes with the air as any other gas would do. And as the sea, lakes, and rivers, are constantly giving off gaseous water into the air in proportion to the temperature, it is not wonderful that the atmosphere always contains gaseous water.
Air is said to be moist when the weight of water in a given quantity, say 100 cubic inches, is as much, or nearly as much, as can exist in the state of gas at the temperature. Under these circumstances, if the temperature is lowered even a very little, some of the gaseous water is converted into liquid water. We see this in hot moist weather, when the outside of a tumbler of fresh drawn cold spring water immediately becomes bedewed. The gaseous water in immediate contact with the tumbler, in fact, is cooled down below the point at which it can all exist as gas, and the superfluity is deposited as dew. In such days wet clothes do not dry well, because there is, already, nearly as much gaseous water in the atmosphere as the amount of heat marked by the thermometer can maintain in that state.
39. When Hot Water is cooled, it Contracts to begin with, but after a time Expands.
We have now seen what a wonderful change is brought about by heating water. At first, it expands gradually and slightly; but, when it reaches the boiling-point, it suddenly expands enormously, and is no longer a liquid, but a gas.
On the other hand, if warm water is allowed to cool, it gradually contracts till it reaches the ordinary temperature of the air in mild weather; but, if the weather is very cold, or if the water is cooled artificially, it goes on contracting only down to a certain temperature (39°), and then begins to expand again. In this peculiarity water is unlike all other bodies which are fluid at ordinary temperatures. Hence the temperature of 39° is that at which pure water has its greatest density or specific gravity, and water at this temperature is heavier, bulk for bulk, than the same water at any other temperature. Therefore if water at the top of a vessel is cooled down to this temperature, it falls to the bottom, and if the water at the bottom of a vessel is cooled below this temperature it rises to the top.