(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.

To render these general principles more intelligible, let us suppose that the water in B is raised to the temperature of 213°, the stopcock C being open; the vessel A will then be filled with steam of the same temperature, and having a pressure of 15 lbs. per square inch. This will be common steam. If the stopcock be now closed, and the whole apparatus be exposed to the temperature of 243°; the steam in A will preserve the same density, but its pressure will be [Pg171] increased from 15 lbs. to a little more than 16 lbs. per square inch. Let the stopcock C be then opened and while the temperature of the steam in A shall continue to be 243°, the pressure will suddenly rise from 16 lbs. to about 26 lbs. per square inch. The weight of the steam in A will be at the same time increased in the same proportion of 16 to 26 as its pressure. The steam thus produced in A will then be common steam, and any abstraction of heat from it would be attended with partial condensation.