(93.)

Since the application of the expansive action of steam involves the consideration of its properties when it ceases to be in contact with the water from which it was produced, and likewise the variation of its pressure in different states of [Pg168] density and at different temperatures, it is necessary here to explain some of the most important of these properties of vapour.

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.