The following table exhibits the density of some of the most important gases, air being taken as a standard :

Hydrogen is the lightest known body, its density being fourteen and a half times less than that of air.

VI.-CHANGE OF STATE OF BODIES BY THE ACTION OF HEAT.

Fusion.

216. It has been stated that heat not only causes bodies to expand, but that it may in certain circumstances cause them to change from the solid to the liquid state, or from the liquid to the gaseous state.

When a body passes from a solid to a liquid state, it is said to melt, or fuse, and the act of changing state in this case is called fusion.

If a melted body is suffered to cool, it becomes solid at the same temperature at which it melted. Hence the melting point is the same as the freezing point.

Fusion takes place when the force of cohesion, which holds the particles of a body together, is exactly balanced by the heat which tends to separate them. The temperature at which fusion takes place is different for different bodies. For some bodies it is very low, and for others very high, as is shown in the following

What is the lightest body? Give the densities of some other gases. (216) What is melting or fusion? When does fusion take place? Is the melting point the same for all solids?

Some are de

Simple bodies,

All bodies are not melted by the action of heat. composed, such as paper, wood, bone, marble, &c. that is, bodies which are composed of but one kind of matter, always melt, if sufficiently heated, with a single exception. Carbon has thus far resisted all attempts to fuse it.

Latent Heat of Fusion.

217. Bodies which can be melted always present the remarkable phenomenon, that when they are heated to the temperature of fusion, they can not be heated any higher until the fusion is complete. For example, if ice be exposed to heat, it begins to melt at 32° F., and if more heat be applied, the melting is accelerated, but the temperature of the mixture of ice and water remains at 32° until all the ice is melted.

The heat that is applied during the process of fusion, enters into the body without raising its temperature, and is said to become latent. When the body returns to its solid state, all the latent heat is again given out, and once more becomes sensible.

The phenomenon of latent heat may be illustrated by the following experiment. If a pound of pulverized ice, at 32° F., be mixed

Examples. Are all bodies melted by the action of heat? Examples. (217.) What is latent heat? Sensible heat? How may the phenomenon of latent heat be illustrated.

with a pound of water at 174° F., the heat of the water will be just sufficient to melt the ice, and there will result two pounds of water at the temperature of 32° F. During the process of melting, 142° of heat have been absorbed and become latent; hence, we say that the heat required to melt ice at 32° F. is 142°, or; in other words, the latent heat of water at 32° is 142°.

The enormous amount of heat which becomes latent when ice melts, explains why it is that large masses of ice remain unmelted for a considerable time after the temperature of the air is raised above 32° F. Conversely, the immense quantity of heat evolved when water passes to the state of ice, explains why it is that ice forms so slowly in extremely cold weather. The absorption of heat in melting, and production of heat in freezing, tend to equalize the temperature of climates in the neighborhood of large masses of water, like lakes and rivers.

Congelation.-Solidification.-Regelation.

218. Any body that can be melted by the application of heat, can be brought back to a solid state by the abstraction of heat. This passage from a liquid to a solid state is called congelation, or solidification.

In every body, the temperature at which congelation commences, is the same as that at which fusion begins. Thus, if water be cooled, it will begin to congeal at 32° F., and conversely, if ice be heated, it will begin to melt at 32° F. Furthermore, the amount of heat given out, or rendered sensible in congealing, is exactly equal to that absorbed, or rendered latent in melting.

Some liquids can not be congealed by the greatest cold to which we can subject them; such are alcohol and ether. Pure water congeals at 32° the salt water of the ocean congeals at 27°; olive oil at 21° linseed and nut oils at 17°.

Explain the action of latent heat on melting masses of ice. Also on freezing masses of water. (218) What is congelation? How does the point of congelation compare with that of fusion? Illustrate. How does the heat given out in solidifying compare with that taken up in melting? What liquids have never been frozen ?

Water reaches its maximum density at 38°. 75, and as its temperature is diminished from this limit, its volume continues to increase until congelation is completed.

If two smooth pieces of melting ice be pressed against each other they are soon frozen together. This phenomenon is called regelation.

Regelation is explained by supposing the interior of the ice colder than the outer layer just passing into the state of water. When the pieces are pressed together the layer of water at 32° F. has a colder body on each side. The latent heat of fusion of this layer is soon absorbed and conducted away, and the water is converted into ice. The formation of a snow-ball depends on regelation. Below a temperature of 32° F. the particles of snow are dry and regelation cannot take place. Hence a coherent snow-ball can only be made of melting snow.

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 sulphur, 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

Explain the phenomenon of regelation. Illustrate. (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 ?