It is because of the expansion of water when heated that larger sizes must be used on flow pipes than on returns, and it is for the same reason that expansion tanks must be used.

• It is due to the expansion and contraction of bodies that circulation takes place, whether it be circulation of water, steam, or air. As water becomes heated, the heated particles are in a more expanded state than the colder particles, and therefore are lighter. They therefore tend to rise, allowing the colder and heavier particles to fall and fill their places. The heating of air results in similar action.

Water is a substance which expands and contracts differently than any other substance. Other substances expand when heated and contract when cooled. At a certain definite temperature, however, water will expand whether heated or cooled. This temperature is 39.2° F., the temperature at which water is the heaviest. If this were not so, in freezing, water in the form of ice would be heavier than the water itself, and ice would be formed at the bottom of a body of water rather than at its surface.

Because of its ability to expand when heated to a high point, or cooled to a low point, water will burst the vessel in which it is contained under either condition; in the former case by freezing, and in the latter by the expansive force of steam. In freezing, the expansive force of water is very great, about 30,000 pounds to the square inch.

The boiling of liquids is a subject of interest. Boiling is the rapid production of elastic bubbles of vapor in the mass of a liquid. When Water is heated at the lower part of a vessel the first bubbles are due to the disengagement of air which the water has absorbed. Small bubbles of vapor then begin to rise from the heated parts of the sides, but as they pass through the upper layers, the temperature of which is lower, they condense before reaching the surface. The formation and condensation of these first bubbles result in the singing noticed in water before it begins to boil. Finally, large bubbles rise and burst at the surface, and it is this action that constitutes boiling.

The temperature at which water will boil, that is, its boiling point, rises with the pressure. Therefore, water will boil more quickly at the top of a mountain than at its base, as atmospheric pressure at the top is less than at the bottom.

For any given pressure, boiling begins at a certain temperature, which for equal pressure, is always the same for the same liquid.

Regardless of the amount of heat that is applied, as soon as boiling commences, the temperature of the liquid remains stationary. Thus, water boils at 212° F. at sea level, and no matter how heavy a heat is being applied, the temperature of the water after it begins to boil remains at 212° F. This is a point worthy of remembrance, as there is a general mistaken idea that the more heat is applied, even after water begins to boil, the higher the temperature of the water will be.

Wafer which contains matter in solution will not boil at as low a temperature as pure water. Thus, a water containing salt, or acid of any kind, will not boil at 212° F. Substances held mechanically by the water, however, such as sediment, sand, etc., do not affect the boiling point.

Evaporation is a subject in which the plumber has an interest, as evaporation is one of the most dangerous evils that the trap seal is subject to.

Evaporation is the slow passage of a liquid into the form of vapor. Whatever may be the temperature at which a vapor is formed, an absorption of heat always takes place. If, therefore, a liquid evaporates, and does not receive from other sources a quantity of heat equal to that which is given out in producing the vapor, its temperature sinks, and the cooling is greater in proportion as the evaporation is more rapid. Experiments have been made, showing that water may be frozen by means of rapid evaporation. Evaporation increases as the temperature increases, and as the exposed surface increases. Currents of air passing over the surface of a body of water also increase the rate of evaporation. The latter cause is the reason that the vent system increases the rate of evaporation of trap seals.

The vent system communicates with the outer air, and it is the currents of air passing through the piping that adds to this evil. The reverse action of evaporation is that of condensation. Condensation is the passage of a vapor into the liquid state. Warm vapors coming in contact with cold surfaces result in the condensation of the vapor. This action underlies the operation of distilling apparatus, the vapor of the liquid that is boiled being condensed again into the form of liquid. Condensation of vapors in the vent system necessitates the running of vent pipes in such a way that the condensation may not accumulate in the pipes, but will drain off into the drainage system. In summer time, the condensation accumulating on cold-water pipes is an evil often encountered.

Heat may be transmitted in two different ways, by conduction and by radiation. When a piece of metal is heated at one point this heat gradually spreads through the metal. When heat is transmitted through the mass of a body, from one particle to another, the body is said to be heated by conduction. Metals are excellent conductors of heat.

Heating by radiation is of more practical importance to the plumber, however, than heating by conduction. When a person stands at a little distance from a fire or other source of heat, he experiences the sensation of warmth. The heat in this case is not transmitted by conduction by the intervening air, but the rays of heat pass through without raising the temperature of the air. This heat is transmitted by radiation. That the heat is not due to the temperature of the air may be known from the fact that if a screen be interposed the sensation at once disappears, which would not be the case if the surrounding air had a high temperature. Therefore, it is seen that bodies may send out rays which excite heat, and which pass through the air without heating it.