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_. (95.) Let us now suppose that the vessel A, being in communication with the vessel B by the open stopcock, has been filled with pure steam of any given temperature. The steam which it thus contains will be common steam, and, as has been [Pg170] shown (94.), it cannot lose any portion of heat, however small, without being partially condensed; but let the stopcock C be closed, and let the steam in A be then exposed to any source of heat by which its temperature may be raised any required number of degrees. From the steam thus obtained heat may be abstracted without producing any condensation; and such abstraction of heat may be continued without producing condensation, until the steam is cooled down to that temperature at which it was raised from the water in B, when the stopcock C was opened. Any further reduction of temperature would be attended with condensation. If after increasing the temperature of the steam in A, the stopcock C being shut so as to render it superheated steam, its pressure be observed, the pressure will be found to be increased, but not to that amount which it would have been increased had the steam in A been raised to the same temperature by heating the water in B to that temperature, and keeping the stopcock open. In fact, its present augmented pressure will be due only to its increased temperature, since its density remains unchanged. But if in these circumstances the stopcock C be suddenly opened, the pressure of the steam in A will as suddenly rise to that pressure which in common steam corresponds to its temperature; and if the vessel A were weighed, it would be found to have increased in weight, proving that the steam contained in it has received increased density by an increased quantity of vapour proceeding from the water in A. In fact, by opening the stopcock the steam which was before superheated steam, has become common steam. It has the greatest density which steam of that temperature can have; and consequently, if any heat be abstracted from it, a partial condensation will ensue.