Ebullition then means a bubbling up or boiling; and when water is heated in an open vessel two forces oppose its conversion into vapour; viz., its own cohesive force and atmospheric pressure. At length, at 212° Fahr., the particles of water have gained by heat a force greater than the opposing forces; bubbles of vapour rise up from the bottom and go off in vapour. This is ebullition, and at that point the tension of the vapour is equal to the pressure of the atmosphere, for if not, the bubbles would not form. All this time of boiling, notwithstanding any increase of heat, the thermometer will not rise above 212° (Fahr.), for all the heat is employed in turning the water to steam.
We have said the ebullition takes place at 212° Fahr. (or 100° C.), but that is only at a certain level. If we ascend 600 feet high we shall find that water will boil at a less temperature; and on the top of a mountain (say Mont Blanc) water will boil at 185° Fahr.; so at an elevation of three miles water boils at a temperature less by 27° Fahr. An increase of pressure similarly will raise the boiling point of water. The heights of mountains are often ascertained by noticing the boiling point of water on their summits, the general rule being a fall of one degree for every 530 feet elevation at medium altitudes. We append a few instances taken at random:—
| Place. | Height above level of the sea—Feet. | Barometer mean height. | Boiling point of water, Fahr. |
|---|---|---|---|
| Quito | 9,541 | 20·75 | 194·2 |
| Mexico | 7,471 | 22·52 | 198·1 |
| St. Gothard | 6,808 | 23·07 | 199·2 |
| Garonne (Pyrenees) | 4,738 | 24·96 | 203·0 |
| Geneva | 1,221 | 28·54 | 209·5 |
| Paris (1st floor) | 213 | 29·69 | 211·5 |
| Sea level | 0 | 30·00 | 212·0 |
[The difference for a degree depends upon the height, varying between 510 and 590 feet, according to the elevation reached. The approximate height of a mountain can be found by multiplying 530 by the number of degrees between the boiling point and 212°. In some very elevated regions travellers have even failed to boil potatoes.]
The boiling point of liquid may be altered by mixing some substance with it; and although such a substance as sawdust would not alter the boiling point of water, yet if the foreign matter be dissolved in the liquid it will alter the boiling point. Even the air dissolved in liquids alters their boiling point, and water freed from air will not boil till it is raised to a temperature much higher than 212° Fahr. Water will boil at a higher temperature in a glass vessel than in metal, because there is a greater attraction between water and glass.
We said above that an increase of pressure will raise the boiling point of water. Under the pressure of one atmosphere—that is, when there is a pressure of 15 lbs. on the square inch—water boils at 212°. But under a pressure of two atmospheres, the boiling point rises to 234°, and of four atmospheres, 294°. So we see by increasing the pressure the water may be almost indefinitely heated, and it will not boil. We can understand that in a very deep vessel the layer of water at the bottom has to sustain the pressure of the water in addition to the weight of the atmosphere above it. The pressure of thirty-four feet of water is equal to the atmospheric pressure of 15 lbs. on the square inch, and thus at such a distance water must be heated to 234° before it will boil. Professor Bunsen founded his Theory of the Geysers upon this fact, for he maintained that water falling into the earth lost much air, and required with the super-incumbent pressure a very high temperature to boil it. When it did boil it generated steam so suddenly that it exploded upwards, throwing up vapour and the water with it, as water poured into a very hot basin will do.
Evaporation may now be considered, and is distinguished from Ebullition by the production of vapour on the surface of liquids, the latter term signifying the formation of vapour in the body of the liquid. Evaporation takes place at all temperatures, and from every liquid surface exposed to the air. We know what we call a “drying wind.” The air in fresh layers continually passing over the wet ground, takes up the moisture; like the east wind, for instance, which has great capabilities of that nature. Damp air can only take up a certain quantity, and when it contains as much water as corresponds to the temperature it can take no more, and is “saturated with moisture”; then evaporation ceases. Heat is a great cause of evaporation, and the greater the surface the more rapid the process, and in a vacuum more readily than in atmospheric air. Evaporation is resorted to very commonly to produce coolness; for instance, the universal fan, by increasing evaporation from a heated skin, generates a feeling of coolness; and we know the vaporization of ether will freeze into insensibility. When a fluid evaporates we can tell that the heat passes away at the same time, for we cool water in porous jars, which permit some of it to pass off in vapour, the remainder being cooled.
Sir John Leslie invented a method of freezing water by rapid evaporation on sulphuric acid under the receiver of an air-pump, and water has been frozen even on a hot plate by these means. By pouring sulphurous acid and water on this plate, the acid evaporates so quickly that it produces sufficient cold to freeze the water it quitted into solid ice.
We leave the phenomena of clouds and watery vapour in the atmosphere for consideration on another opportunity, under the head of Meteorology, Rain, etc.