(94.)

We shall call, for distinction, the former Common Steam, and the latter Superheated Steam.

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]

The increase of temperature which it has received would of itself produce an increased pressure; but that this is not the sole cause of the augmented pressure in the present case might be proved by weighing the vessel A. It would be found to have increased weight, which could only arise from its having received from the water in B an additional quantity of vapour. The increased pressure therefore, which the steam in A has acquired, is due conjointly to its increased density and its increased temperature. In general, if the water in the vessel B be raised or lowered in temperature, the steam in the vessel A will rise and fall in temperature in a corresponding manner, always having the same temperature as the water in B. If the weight of the vessel A were observed, it would be found to increase with every increase of temperature, and to diminish with every diminution of temperature, proving that the augmented temperature of the water in B produces an augmented density of the steam in A. The same pressure would be found always to correspond to the same temperature and density, so that if the numerical amount of any one of the three quantities, the temperature, the pressure, or the density, were known, the other two must necessarily be determined, the same temperature always corresponding to the same pressure, and vice versâ. And in like manner, steam produced under these circumstances of the same density cannot have different pressures. It must be observed that the steam here produced receives all the heat which it possesses from the water from which it is raised. Now it is easily demonstrable, that this is the least quantity of heat which is compatible with the steam maintaining the vaporous form; for if the stopcock C be closed so as to separate the steam in A from the water in B, and that any portion of heat, however small, be then abstracted from the steam in A, some portion of the steam will be reconverted into water.

This then, according to the definition already given, is Common Steam.