_Ebullition or Boiling of Water, Steam._--The atmosphere around us is composed of a mixture of nitrogen and oxygen gases; not a compound of these gases, as water is of hydrogen and oxygen, but a mixture more like sand and water or smoke and air. This mass of gases has weight, and presses upon objects at the surface of the earth to the extent of 15 lb. on the square inch. Now some liquids, such as water, were it not for this atmospheric pressure, would not remain liquids at all, but would become gases. The pressure thus tends to squeeze gases together and convert them into liquids. Any force that causes gases to contract will do the same thing, of course--for example, cold; and _ceteris paribus_ removal of pressure and expansion by heat will act so as to gasify liquids. When in the expansion of liquids a certain stage or degree is reached, different for different liquids, gas begins to escape so quickly from the liquid that bubbles of vapour are continually formed and escape. This is called ebullition or boiling. A certain removal of pressure, or expansion by heat, is necessary to produce this, _i.e._ to reach the boiling-point of the liquid. As regards the heat necessary for the boiling of water at the surface of the earth, _i.e._ under the atmospheric pressure of 15 lb. on the square inch, this is shown on the thermometer of Fahrenheit as 212 deg., and on the simpler centigrade one, as 100 deg., water freezing at 0 deg. C. But if what I have said is true, when we remove some of the atmospheric pressure, the water should boil with a less heat than will cause the mercury in the thermometer to rise to 100 deg. C., and if we take off all the pressure, the water ought to boil and freeze at the same time. This actually happens in the Carre ice-making machine. The question now arises, "Why does the water freeze in the Carre machine?" All substances require certain amounts of heat to enable them to take and to maintain the liquid state if they are ordinarily solid, and the gaseous state if ordinarily liquid or solid, and the greater the change of state the greater the heat needed. Moreover, this heat does not make them warm, it is simply absorbed or swallowed up, and becomes latent, and is merely necessary to maintain the new condition assumed. In the case of the Carre machine, liquid water is, by removal of the atmospheric pressure, coerced, as it were, to take the gaseous form. But to do so it needs to absorb the requisite amount of heat to aid it in taking that form, and this heat it must take up from all surrounding warm objects. It absorbs quickly all it can get out of itself as liquid water, out of the glass vessel containing it, and from the surrounding air. But the process of gasification with ebullition goes on so quickly that the temperature of the water thus robbed of heat quickly falls to 0 deg. C., and the remaining water freezes. Thus, then, by pumping out the air from a vessel, _i.e._ working in a vacuum, we can