CHAPTER V.

I. GENERAL

HEAT.

PROPERTIES OF HEAT.

Definition of Heat.

176. HEAT is a physical agent capable of exciting in us the sensation which we call warmth. Absence of heat constitutes cold.

Theories of Heat.

177. Two principal theories have been advanced to explain the phenomena of heat.

According to the first theory, heat is a fluid, destitute of weight, and capable of flowing from one body to another. The particles of this fluid mutually repel each other, and are attracted by the particles of other bodies. This is called the emission theory.

According to the second theory, heat consists of a vibratory motion of the particles of bodies, which motion is transmitted from one body to another through an elastic fluid, called ether, in the same manner that sound is transmitted through air. The warmest bodies are those in which these vibrations are most rapid and most extensive. This is the undulatory theory.

According to the emission theory, a body cools by losing

(176.) What is Heat? Cold? (177.) What two theories of heat have been advanced? Explain the emission theory. The undulatory theory.

a portion of the fluid; according to the undulatory theory, they simply lose a part of their motion.

In the progress of science, the undulatory theory seems to be growing into favor, but for the purposes of explanation, the emission theory is preferable.

General Effects of Heat.

178. Heat, accumulating in bodies, penetrates into their substance, and uniting with their ultimate molecules, gives rise to repellent forces which counteract those of cohesion. Hence, the most noticeable phenomenon of heat is, that it causes bodies to expand. If applied in sufficient quantity, the particles of solids are so far repelled, as to move freely amongst each other, becoming liquid; or if still greater quantities of heat are applied, the body passes into a state of vapor. When heat is abstracted from a vapor, it returns to a liquid state, and if still more heat be abstracted, it becomes solid, and if the process be continued, the solid goes on contracting under the influence of the molecular forces.

Hence we say, that heat dilates bodies, and cold contracts them. Heat also converts solids into liquids, liquids into vapors, and acting upon gases and vapors, causes them to expand.

Expansion of Bodies by Heat.

179. All bodies are expanded by heat, but in very different degrees. The most dilatable bodies are gases, then vapors, then liquids, and finally solids. In fluids we regard only increase of volume, but in solids we distinguish two kinds of expansion, linear expansion, that is, expansion in length, and expansion of volume.

(178.) Describe the general effects of heat on solids. On liquids. What effect has cold on vapors? On liquids? (179.) What bodies are most dilatable? The least dilatable? What is linear expansion? Expansion of volume?, ¦

Fig. 126 represents the method of showing and measuring the linear expansion of the metals. A rod of metal, A, passes through two metallic supports, being made fast at one extremity by a clampscrew, B, and being free to expand at the other extremity. The free end abuts against the short end, C, of a lever, the long end, D, of which plays in front of a graduated arc.

When the rod is heated, by placing fire beneath it, as shown in the figure, the rod A expands, and the expansion is shown by the

motion of the index, D. When the rod, A, is of steel, copper, silver, &c., the amount of expansion varies, as is shown by the different amounts of displacement of the index. Brass, for example, expands more, for the same amount of heat, than iron or steel.

Fig. 127 shows the method of demonstrating that bodies undergo an expansion in volume when heated. A ring, A, is constructed so that a ball, B, passes freely through it when cold. If the ball be heated in a furnace, it will no longer pass through the ring, but if allowed to cool, it again falls through the ring. The method of making the experiment is fully shown in the figure.

Liquids and gases being more expansible than solids, their expansion is more easily shown by experiment. For liquids, we take a hollow glass sphere, terminating in a narrow tube, open at the top, and fill the globe and a portion of the stem with some fluid, like mercury, as shown in Fig. 128. If heat be applied to the globe, the liquid will rise in the stem from a towards b, indicating an increase of volume; and if sufficient heat be applied, the liquid will fill the stem, and will ultimately be converted into vapor. If the liquid is allowed to cool, it again returns to its original volume.

An analogous experiment shows the expansion of gases and vapors. A bulb of glass is provided with a long and fine tube of the same material, which is bent twice upon itself, as shown in Fig. 129. An index of mercury is introduced into the stem in the following manner. The bulb is heated, and a portion of the air which it contains is driven

m

Fig. 128.

Fig. 129.

How is expansion in volume shown? How is the expansion of liquids shown? Of gases?

out, when a drop of mercury is poured into the funnel, a. If the: instrument is allowed to cool, the air in the bulb contracts, and the pressure of the atmosphere drives the drop of mercury along the tube to some position, m.

The instrument having been prepared in this manner, if the bulb is held in the hand for a few minutes, the air becomes heated and expands, the expansion being indicated by the index moving to some new position, as n. If allowed to cool, the index returns to m.

From what precedes, we infer that heat expands all bodies, and that cold contracts them. There are apparent exceptions to this law, but they are only apparent. Thus, bodies capable of absorbing water, like paper, wood, clay, and the like, contract on being heated. This contraction is only apparent; it arises from the water which they contain being vaporized and driven off, which produces an apparent diminution of volume; after they are thoroughly dried, they follow the general law.

The phenomenon just explained is used for bending absorbent bodies. To effect this they are heated on one side only, which drives out the water from that side, and causes them to bend in that direction. It is this principle that causes wooden articles to warp, and therefore demands that articles of furniture, and wooden parts of buildings, be coated with oils, paints, or varnishes, to prevent the absorption of water.

The principle of expansion and contraction is often utilized in the arts.

A familiar example, is the process of setting the tire of a wagonwheel. The tire is made a little smaller than the outer periphery of the wooden part of the wheel. It is then heated, and placed around the wheel; on cooling, it contracts powerfully, and draws the felloes firmly together, The same principle has been applied

What is the general conclusion with respect to the action of heat and cold? Explain the apparent exceptions to the law. Explain the process of warping Are the principles of contraction and expansion utilized? Explain the operation of setting a tire. Qf drawing walls together.