Let us now consider the Effects of Heat upon Water. We have all seen the vapour that hangs above a locomotive engine. We call it “steam.” It is not pure steam, for steam is really invisible. The visible vapour is steam on its way to become water again. On a very hot dry day we cannot distinguish the vapour at all.
The first effect of heat upon water is to expand it; and as the heat is applied we know that the water continues to expand and bubble up; and at last, when the temperature is as high as 212°, we say water “boils”—that is, at that heat it begins to pass away in vapour, and you will find that the temperature of the steam is the same as the boiling water. While undergoing this transformation, the water increases in volume to 1,700 times its original bulk, although it will weigh the same as the water. So steam has 1,700 less specific gravity than water.
It is perhaps scarcely necessary to remind our readers that water, when heated, assumes tremendous force. Air likewise expands with great violence, and the vessels containing either steam or air frequently burst, with destructive effects. Solid bodies also expand when heated, and the most useful and accurate observations have been made, so that the temperatures at which solid bodies expand are now exactly known. Air also expands by heat.
While speaking of Expansion by Heat, we may remark that a rapid movement is imparted to the air by Heat. In any ordinary room the air below is cool, while if we mount a ladder to hang up a picture, for instance, we shall find the air quite hot near the ceiling. This is quite in keeping with the effects of heat upon water. The hot particles rise to the top in a vessel, and thus a motion is conveyed to the water. So in our rooms. The heated air rushes up the chimney and causes a draught, and this produces motion, as we have seen by fig. 39, in which the cardboard spiral was set in motion by heated air. A balloon will ascend, because it is filled with heated air or gas; and we all have seen the paper balloons which will ascend if a sponge containing spirit of wine be set on fire underneath them.
Winds are also only currents of air produced by unequal temperature in different places. The heated air ascends, and the colder fluid rushes in sometimes with great velocity to fill the space. “Land” and “sea” breezes are constant; the cool air blows in from the sea during the day, and as the land cools more rapidly at night, the breeze passes out again. When we touch upon Meteorology, we will have more to say respecting Air Currents and the various Atmospheric Phenomena.
We know that water can be made to boil by heat, but it is not perhaps generally known that it will apparently boil by cold, and the experiment may thus be made:—A flask half-full of water is maintained at ebullition for some minutes. It is removed from the source of heat, corked, inverted, and placed in one of the rings of a retort stand. If cold water is poured on the upturned bottom of the flask, the fluid will start into violent ebullition. The upper portion of the flask is filled with steam, which maintains a certain pressure on the water. By cooling the upper portion of the flask some of this is condensed, and the pressure reduced. The temperature at which water boils varies with the pressure. When it is reduced, water boils at a lower heat. By pouring the cold water over the flask we condense the steam so that the water is hot enough to boil at the reduced pressure. To assert that water boils by the application of cold is a chemical sophism.
Ebullition and Evaporation may be now considered, and these are the two principal modes by which liquids assume the gaseous condition. The difference is, when water boils we term it ebullition (from the Latin ebullio, I boil); evaporation means vapour given out by water not boiling (from evaporo, I disperse in vapour).
There are two operations based upon the properties which bodies possess of assuming the form of vapour under the influence of heat, which are called Distillation and Sublimation. These we will consider presently.
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