_Steam_ is water in a gaseous state; the gas or vapor of water; it
liquifies under a pressure of 14·7 and temperature of 212° F.
_Steam_ is a joint production of the intermingling of water and heat.
Water is composed of two gases which have neither color nor taste, and
steam is made up of the same two gases with the addition only of that
mysterious property called heat by which the water becomes greatly
expanded and is rendered invisible. The French have a term for steam
which seems appropriate when they call it water-dust.
This is what takes place in the formation of steam in a vessel
containing water in free communication with the atmosphere. At first,
a vapor is seen to rise that seems to come from the surface of the
liquid, getting more and more dense as the water becomes hotter. Then
a tremor of the surface is produced, accompanied by a peculiar noise
which has been called _the singing_ of the liquid; and, finally,
bubbles, similar to air bubbles, form in that part of the vessel which
is nearest to the fire, then rise to the surface where they burst,
giving forth fresh vapor.
The curious fact must be here noted that if water be introduced
into a space entirely void of air, like a vacuum, it vaporizes
instantaneously, no matter how hot or cold, so that of an apparent and
fluid body there only remains an invisible gas like air.
That steam is _dry_ at high pressure is proved by an experiment which
is very interesting. If a common match head is held in the invisible
portion of the steam jet close to the nozzle, it at once lights, and
the fact seems convincing as to complete dryness, as the faintest
moisture would prevent ignition even at the highest temperature. This
experiment proves dryness of the steam at the point of contact, but if
throttling exists behind the jet, the steam supplied by the boiler may
be in itself wet and dried by wire drawing.
_Dead steam_ is the same as exhaust steam.
_Live steam_ is steam which has done no work.
_Dry steam_ is saturated steam without any admixture of mechanically
suspended water.
_High-pressure steam_ is commonly understood to be steam used in high
pressure engines.
_Low-pressure steam_ is that used at low pressure in condensing
engines, heating apparatus, etc., at 15 lbs. to the inch or under.
_Saturated steam_ is that in contact with water at the same
temperature; saturated steam is always at its condensing point, which
is always the boiling point of the water, with which it is in contact;
in this it differs from superheated steam.
_Superheated steam_, also called steam-gas, is steam dried with heat
applied after it has left the boiler.
_Total heat of steam_ is the same as steam heat.
_Wet steam_, steam holding water mechanically suspended, the water
being in the form of spray.
Specific gravity of steam is ·625 as compared to air under the same
pressure.
The properties which make it so valuable are: