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.