(93.)
Steam may exist in two states, distinguished from each other by the following circumstances:—
1st. It may be such that the abstraction from it of any portion of heat, however small, will cause its partial condensation.
2d. It may be such as to admit of the abstraction of heat from it without undergoing any other change than that which air would undergo under like circumstances, viz. a diminution of temperature and pressure.
(94.)
Fig. 29.
To explain the circumstances out of which these properties arise, let B ([fig. 29.]) be imagined to be a vessel filled with water, communicating by a pipe and stopcock with another vessel A, which in the commencement of the process may be conceived to be filled with air. Let D be a pipe and stopcock at the top of this vessel. If the vessel B be heated, and the two cocks be opened, the steam proceeding from the water in B will blow the air out of the vessel A through the open stopcock D, in the same manner as air is blown from a steam engine. When the vessel A by these means has been filled with pure steam, let both stopcocks be closed. If the steam in A, under these circumstances, have a pressure of 15 lbs. per square inch, its temperature will be found to be 213°. Now, if any heat be abstracted from this steam, its temperature will fall, and a portion of it will be reconverted into water.
Again, suppose the vessel A to be filled with pure steam which has been produced from the heated water in B, the stopcock C being open. Let the stopcock C be then closed, and the water in B be heated to a higher temperature, the temperature and pressure of the steam in A being observed. If the stopcock C be now opened, the steam in A will be immediately observed to rise to the more elevated temperature which has been imparted to the water in B, and at the same time it will acquire an increased pressure. [Pg169]