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:
1. The ease with which we can condense it.
3. The small space in which it shrinks when it is condensed either in a vacuum chamber or the air.
A cubic inch of water turned into steam at the pressure of the atmosphere will expand into 1,669 cubic inches.
The Davidson pump is shown complete in Fig. 248; the valve motion consists principally of _a valve, valve pistons, valve pin and cam_. The main valve is operated by a positive mechanical connection between it and the main piston rod, also by the action of steam on the valve pistons. The engraving, Fig. 249, shows the details of valve gear and steam cylinder. The steam end consists of the cylinder, M, valve, A, and valve pistons, B and B. These pistons are connected with sufficient space between them for the valve, A, covering the steam ports, F and F_{ 1}, Fig. 250.