Water reaches its maximum density at 38°. 75, and as its temper. ature is diminished from this limit, its volume continues to increase until congelation is completed. This increase of volume takes place with an expansive force capable of bursting the strongest vessels. Hence, in cold weather, water should not be left in vessels and pipes, or in any apparatus which might be broken by frost.

On account of this expansion of water in congealing, it follows that it is less dense than before; hence it is that ice floats on the surface of water. In the polar regions immense masses of floating ice, called icebergs, are continually seen. Some of these are of immense height, and extend to a corresponding depth in the water. In passing shoal places, they often become stranded, and remain fixed until, by gradual melting, their volume is sufficiently reduced to permit them to float clear of the bottom.

Crystallization.

219. When bodies pass slowly from the liquid to the solid states, their particles, instead of arranging themselves in a confused manner, tend to group themselves into regular forms. These forms are called crystals, and the process of forming them is called crystallization.

Flakes of snow, sugar candy, alum, common salt, and the like, offer examples of crystallized bodies. The forms of the crystals are best seen under a magnifying glass.

Bodies may be crystallized in two different ways. In the first case, we melt them, and then allow them to cool slowly. If a vessel of sulphur be melted and allowed to cool slowly, it will commence crystallizing about the surface, and if we break the crust thus formed, and pour out the interior liquid fulphur, we may obtain beautiful crystals of sulphur.

In the second case, we dissolve the body to be crystallized and then allow the solution to evaporate slowly. The dissolved body is then deposited at the bottom and on the

Why are vessels often burst by the freezing of water in them? Precautions? Why does ice float? Explain the phenomenon of icebergs. (219.) What are crystals? What is crystallization? Examples. How many methods of crystallization? Explain the first method? The second method?

sides of the vessel in the form of crystals. The slower the process, the finer will be the crystals. It is in this manner that we crystallize candy and various salts.

Freezing Mixtures.

220. The absorption of heat which takes place when a body passes from a solid to a liquid state, is often utilized in the production of intense cold. This result is best obtained by mixing certain substances, and these mixtures are then called freezing mixtures.

A mixture of one part of common salt and two parts of pounded ice forms a mixture that is used for freezing cream. The salt and ice have an affinity for each other, but they can not unite until they pass to the liquid state; in order to pass to this state they absorb a great quantity of heat from the neighboring bodies, and this causes the latter to freeze. By means of a mixture of salt and snow, the thermometer may be reduced to 0.

VII. VAPORIZATION.-ELASTIC

FORCE OF

VAPORS.

Vaporization. - Volatile and Fixed Liquids.

221. When sufficient heat is applied to a liquid, it is converted into a gaseous form and is called a vapor. The change of state from a liquid to a gaseous state is called vaporization.

Conversely, if heat be abstracted from a vapor, it will return to a liquid form. The change of state from a vaporous to a liquid form is called condensation.

Vapors are generally colorless, and are endowed with an expansive force, or tension, which, when heated, may become very great.

(220.) What is a freezing mixture? Example. Explain its action? (221.) What is vaporization? Condensation? General properties of vapors?

The number of vapors that exist at ordinary temperatures is very small. Of these, watery vapor is the most familiar, as well as the most important, on account of the part which it plays in many natural phenomena.

Liquids are divided into two classes, with respect to the readiness with which they pass from the liquid to the vaporous state, viz.: volatile liquids and fixed liquids.

VOLATILE LIQUIDS are those which have a natural tendency to pass into a state of vapor even at ordinary temperatures, such as ether, alcohol, and the like. If a vessel of water, alcohol, ether, or chloroform be left exposed to the air, the liquid is slowly converted into vapor, and disappears; in other words, it evaporates. To the class of volatile liquids belong essences, essential oils, volatile oils, amongst which may be mentioned spirits of turpentine, oil of lavender, attar of roses, oil of orange, and the like.

FIXED LIQUIDS are those which do not pass into vapor at any temperature, as, for example, fish oils, olive oils, and the like. At high temperatures they are decomposed, giving rise to various kinds of gases, but to no true vapors that can be condensed into the original form of the liquid. Some oils, like linseed oil, harden on exposure to the air, but it is not by evaporation, but by absorbing oxygen from the air, and thus passing to a solid state. Some solids are capable of passing directly to a state of vapor without first becoming liquid. To this class belong camphor, musk, and odorous bodies generally. Snow and ice may, under certain circumstances, evaporate without melting.

Evaporation under pressure.

222. The influence of evaporation by pressure may be illustrated

What are Examples.

The most important vapor? What two classes of liquids have we? volatile liquids? Examples. Illustrate. What are fixed liquids? Effect of high temperatures upon them? Give examples of solids that vaporize?

by means of an apparatus shown in Fig. 145. It consists of a curved tube, the short branch of which is closed and filled with mercury; the mercury also fills a portion of the long branch. A small quantity of ether is introduced into the short branch, when it at once rises to the top, B, of this branch. At ordinary temperatures, the pressure of the external atmosphere exerted through the mercury, is sufficient to prevent the ether from forming vapor

If, however, the tube is plunged into a vessel of water heated to 112, the ether will be converted into vapor and will occupy a certain portion, AB, of the tube, holding in equilibrium the pressure of the atmosphere, together with the weight of the mercurial column whose height is AC.

If the tube be withdrawn and allowed to cool, the vapor of ether will be condensed, and will appear as a liquid at B. If more heat be applied, it will again be converted into vapor, and the mercury will rise in the branch, C, as long as any ether remains to be evaporated. This shows that the tension of the vapor augments with the temperature. This principle holds true for all kinds of vapor.

The tension acquired by the vapor of water, or steam, often be

C

A

comes so great by being heated as

Fig. 145.

to burst the strongest vessels, and

thus is the cause of frightful accidents. The cause of wood snapping when burned in a fire-place, is the expansion of the water in the pores, giving rise at last to an explosion. When a chestnut is roasted in the ashes, the moisture within the shell expands into

(222.) Explain the experiment showing the influence of pressure on vaporization. Why does wood snap when burned? Why does a chestnut snap when roasted?

steam, and explodes with sufficient force to throw the nut from the fire. Hence it is that a small puncture is usually made in the shell, which permits the escape of the steam and prevents explosion

Instantaneous Evaporation in a Vacuum.

223. Vapors formed upon the surface of a liquid escape by virtue of their tension. Under ordinary circumstances, the pressure of the

air prevents a very rapid escape of va por at ordinary temperatures, but when the atmospheric pressure is diminished in any way, evaporation takes place with great rapidity. If the pressure is entirely removed, the evaporation is instantaneous, like the flash of gunpowder, especially if the liquid is very

volatile.

This principle may be illustrated by means of the apparatus shown in Fig. 146.

Fig 146.

It consists of several barometer tubes, A, B, C, D, filled with mercury, and inverted in a common cistern of mercury, as shown in the figure. The whole apparatus is supported by a frame, to which is

How remedied? (223.) Why do vapors escape from the surfaces of liquids? When the pressure is removed, what happens? How may the principle be illus trated? Explain the experiment in detail.