the same height in a close room from which all communication with the external air has been cut off. The lowest stratum of the atmosphere is pressed upon by the strata above it, and being highly elastic, it assumes the condition of a bent spring. The confined air of the room is therefore able to support thirty inches of mercury by the elasticity which it acquired before the doors and windows were closed.
We shall now be able to understand the relation that subsists between the phenomenon of ebullition and atmospheric pressure. Water evaporates, or is converted into steam at all temperatures, until the whole space above it is filled with watery vapour of a certain elasticity. This is a wise provision of nature, for if water obstinately retained its liquid form at all temperatures below 212°, the moisture that descended to the earth in the form of rain would never be evaporated during the hottest summers. But there is a difference between evaporation at low temperatures and ebullition or boiling. Water must be heated until its vapour acquires an elasticity equal to that of the atmosphere before ebullition can take place. At 212° the elastic force of steam will support a column of mercury thirty inches high, and at this temperature the steam-bubbles acquire the power of breaking through the surface of the heated water, provided the barometer stands at thirty inches.
Were we to carry our kettle to the summit of a
