or simmering. When the water becomes sufficiently heated the bubbles increase in number and grow larger as they ascend, break forth at the surface and disappear into the atmosphere. The whole mass of liquid is agitated. There is a certain amount of characteristic noise and the quantity of water diminishes until it is changed completely into steam. In other words the water has been boiled away. Steam is invisible. What is frequently called steam is simply vapor, the minute particles of water arising from condensation of steam, such as the vapor from a kettle. Experiments verify the following law of ebullition.

Every liquid begins to boil at a certain temperature which remains the same until the liquid is entirely vaporized, considering the pressure to remain constant. When cooling the condensation takes place at the same temperature as that at which it changed into the gaseous form.

An increase of pressure raises the boiling point. A decrease of pressure lowers the boiling point. Water, as we know, usually boils at 100° C. or 212° F. under ordinary atmospheric conditions.

FIG. 2.

If we go to the top of a mountain the atmospheric pressure is less because there is not so much air above pressing down on the surface of the liquid, and hence the water boils at a lower temperature. If we close tightly the vessel in which the boiling takes place the pressure increases as the gas is formed, and thus the temperature of boiling is raised.

An illustration of this principle is shown by the following experiment. If we heat a vessel of water to boiling, then stop up the opening from which the steam escapes, and remove it from the source of heat, it stops

boiling. If now we pour cool water over the top of the vessel, Fig. 2, so that the steam contained in it is condensed back to

water by lowering its temperature, the pressure is decreased by the partial vacuum thus formed, and the water, without any addition of heat, commences to boil again until a sufficient pressure is attained to prevent further ebullition.

We have already noted that substances in the gaseous form occupy far more space than they do in the liquid form. Water in the form of steam occupies nearly 1,700 times as much space as it does in the liquid form, hence a cubic inch of water will occupy when converted into steam nearly a cubic foot of space, the pressure being considered in each case to be one atmosphere. The following is a table of boiling points under a pressure of one atmosphere.

Distillation is the process of vaporizing a liquid in a heated vessel and then condensing that vapor in a cool vessel. In one

vessel the substance changes from a liquid to a gas and in the other vessel, from a gas to a liquid. It is chiefly used for the separation of a liquid from a solid held in solution, or for separating a mixture of two liquids which have different boiling points. The process depends upon the fact that different substances are vaporized at different temperatures.

The apparatus which is called a still has many forms, but consists essentially of two parts, a retort for producing the vaporization and a condenser for condensing the vapor back to the liquid form. This is shown in Fig. 3.

Suppose we wish to separate water from salt which is held in solution. The brine is placed in the retort, Fig. 4, and heated a little above 212° F. At this temperature the water changes to steam, but the salt is not vaporized. The steam passes from the

retort through a worm where it is condensed rapidly and runs out into a vessel placed to receive it. The salt of course remains in the retort. The water around the worm is kept cool by feeding it with cold water at the bottom and drawing the heated water away at the top.

Another form of apparatus which is generally used in the laboratory is shown in Fig. 5. This consists of a flask for the retort, a tube passing through a water jacket for the worm and another flask, or some other vessel to receive the results of condensation.

Suppose we wished to separate two liquids such as alcohol and water. The retort in which the solution is placed is heated to about 90° C., which is above the boiling point of alcohol but below that of water. The alcohol passes over as vapor while the water

remains behind. In actual practice a certain amount of steam passes over with the alcohol vapor, and the alcohol vapor and steam pass through another vessel surrounded with boiling water, thus

FIG. 5.

condensing out the steam, while the alcohol which does not condense at so high a temperature passes on.

This process is called fractional distillation. In order to obtain alcohol which is very nearly pure, or absolute alcohol as it is called, the distillation is repeated several times.

CONDUCTION.

If we hold one end of a bar of iron in the fire the end which we hold soon becomes hotter than we can bear. This is because it has been heated by conduction, the molecules first heated giving some of their heat to those adjacent and thus passing it on to those beyond, i. e., there is a transfer of motion from molecule to molecule. This process by which heat passes from the hotter to the colder parts of a body is called conduction of heat. It is gradual and does not depend upon the shape of the bar, whether it is crooked or straight.

If we use a rod of glass, the end in the fire would melt but the other end would not become very warm. If we use a wooden stick, the end in the fire would burn and but very little heat would be transferred to the end held. Thus we see that some substances are good conductors of heat, while others are not.

If a bar of iron and one of copper of equal lengths be placed end to end and heated equally at the point of junction, the farther end of the copper bar will become hot before the farther end of the iron bar. The following table gives the relative thermal conductivity of a few metals.

NOTE. The above metals would be arranged in the same order according to the conduction of the electric current, and thus a relation is suggested between heat and electricity.

Liquids and aeriform bodies are poor conductors of heat. Mercury which is a metal is the only fluid which may be readily heated throughout.

CONVECTION.

Water should be heated from below; the heated molecules expand and rise, the cooler ones descend to take their place nearest the source of heat. In this way currents are produced in the water.

Suppose we have a tube whose sides form a rectangle, and a portion of the water in it is colored by some coloring matter. Then it we heat one side of the rectangle the water on that side begins to rise, while the water from the side of the rectangle away from the heat descends and flows across to take the place of the water which has risen. The colored part of the water shows the movement or the current. The same thing takes place in any vessel containing water, the water rising in one part (usually the center) and descending in other parts (usually those parts next to the sides of the vessel). In this way the water becomes heated throughout. This method of diffusing heat by actual motion of heated fluid masses is called convection. It is the result of expansion by heat and the action of the force of gravity.

Convection takes place also in gases; it takes place in the air thereby giving rise to many phenomena, such as wind, rain, etc.