used. It is in this form that electricity is applied for artificial lighting.

THERMO-ELECTRICITY.

75. An Electric Current produced by Heat.-As the electric current produces heat, so we may invert the order of the phenomena, and from heat derive electricity. When any two metals, unequally susceptible to heat, are soldered together, and heated at the junction, an electric current is evoked. The two metals which show this property most readily are antimony and bismuth. Thus,

B

Fig. 18.

if a bar of antimony, A (fig. 18), be soldered to a bar of bismuth, B, and their other ends connected with a galvanometer, G, a current will pass when we heat the junction, S. Inside the couple, it flows from bismuth to antimony, and outside from antimony to bismuth, as we know by the side to which the needle swings. If cold instead of heat be applied at the junction, it will produce a current opposite in direction to what heat would. One antimonybismuth pair has little power, but several may be joined as in a galvanic battery. The strength of the electric current thus produced by difference of temperature is very weak compared with that due to chemical action. It is as a measurer of heatthat is, as a thermometer-that the thermo-electric battery has its chief importance. Twenty or thirty antimony-bismuth pairs compactly put together are inclosed in an insulating tube, and the ends are blackened to increase the absorption of heat. Wires connect the end plates with a galvanometer. The mere approach of the hand or of a piece of ice is sufficient to deflect the needle, so sensitive is the instrument. For experiments on radiant heat it now supersedes all others.

41

НЕАТ.

76. The Extensive Part it plays.-Heat, as we have seen, is that form of energy into which all the other kinds are oftenest and most readily transformed. No one branch of knowledge, perhaps, gives the key to so many of the appearances of nature as a knowledge of the laws of heat. In all that concerns weather and climate, for instance, heat plays the chief part. It therefore claims here our special attention.

77. Expansion by Heat.-What heat is has been already considered (see par. 43). One of its most marked effects is expansion. As a rule, all bodies increase in bulk when their temperature is increased. Solids expand

least, gases most, and liquids to an intermediate degree. Heat an iron rod in the fire; it will be found sensibly longer and thicker than when cold. Take a glass flask, fill it with water or any other liquid, and apply a spiritlamp; the liquid will soon begin to overflow. If a bladder three-fourths filled with air is held near a fire, it soon becomes quite stretched, and may be made to burst.

78. Differences in Expansibility.-Nearly every solid and liquid has an expansibility peculiar to itself. Among solids, the metals are the most expansible. Zinc expands most, platinum probably least among bodies of the metallic class. Glass, brick, porcelain, marble, and stone have small expansibilities. If a rod of iron which measures 819 lines in length when as cold as melting ice, is made as hot as boiling water, it is found to measure 820 lines. Between the freezing and boiling points, then, iron increases of its length; for the same increase of heat, glass expands only

of its length. Gases, unlike solids and liquids, have not specific expansibilities, but all undergo almost the same amount of expansion for the increase of temperature.

When heated from 32° to 212°, mercury dilates

of

its bulk, and alcohol; air or any other gas, about 1. An increase of 1° of temperature, therefore, increases a body of air by of its bulk.

1

490

79. Exceptional Case of Water.-Water presents a singular irregularity in its expansions and contractions. If boiling water is allowed gradually to cool, it follows the general law, and goes on contracting until it is within a few degrees of freezing (at 39°); it then begins to dilate, and continues to do so till it come to 32°, the freezing-point. At the moment of becoming solid, it undergoes a sudden enlargement. It is this enlargement of freezing water that causes it to burst pipes and vessels in which it is confined; it is also the reason that ice is lighter than water, and floats on the surface. Ten cubic inches of ice weigh as much as nine cubic inches of water.

In all artificial structures, especially when metal is used, allowance has to be made for the expansion and contraction of the parts through changes of temperature. But its effects in nature concern us most here. All the more important movements of the atmosphere and the currents of the ocean primarily arise from expansions and contractions through heat and cold (pars. 178, 179, 202); and to the exceptional behaviour of water, we owe it that the temperate regions of the globe are habitable (see par. 248).

80. Rationale of Expansion.-The mechanical theory of heat-that is, the theory that it is a mode of motion -accounts for the phenomena of expansion. If heat consists of vibrations of the particles of matter, to in

crease it is to increase the rapidity and width of the vibrations. Thus the particles are made to urge one another farther apart, and the volume of the body is increased. The expansion of freezing water is accounted for by the hypothesis that the crystalline arrangement of the molecules occupies more room than the liquid arrangement. Some other substances expand in solidifying, notably iron and sulphur.

THE THERMOMETER.

81. Its Construction.-The thermometer is an instrument in which temperature—that is, the intensity of heat -is measured by the amount of expansion it produces. The most convenient substances for the construction of thermometers are found to be mercury and alcohol, or spirit of wine. For ordinary temperatures, mercury is preferable; but since it freezes at very low temperatures, alcohol is used, which cannot be congealed by any cold we can produce. The mercury or alcohol is inclosed in a glass tube with a hollow bulb at one end, the other being closed. It is then graduated by first plunging it in melting ice, and marking on the glass, or on an ivory scale attached to it, the point at which the mercury stands. This mark is the freezing-point of water; for water freezing and ice melting have the same temperature. The thermometer is next placed in the steam arising from boiling water, when the barometric pressure is 29.905 inches. A second mark is here made, which is

the boiling-point.

The space between these two points is then divided into a number of equal parts, called degrees, and parts of the same length are set off above and below the boiling and freezing points, as far as required.

82. Thermometers of Different Kinds.-In the ther

D

mometer mostly used in this country, the space between

F

212

192

C

100

80

60

40

172

152

132

112

92

72

52

32

20

Fig. 19.-Thermometers.

the freezing and boiling points is divided into 180 equal parts, and we begin counting at 32° below the freezing-point. A cipher is placed there, and it is called the zero or nothingpoint of the thermometer. The freezing-point of water thus comes to be marked by the number 32°, and the boilingpoint, which is 180° higher, by 212°. In the thermometer chiefly used on the continent, the space between the freezing and boiling points of water is divided into 100 equal parts, and the gradu

F, Fahrenheit; C, Centigrade. ation begins at the freezingpoint, which is marked 0°, or zero. thermometer, which is called the freezes at 0°, and boils at 100°.

According to this
Centigrade, water

The centigrade thermometer is now much employed in scientific researches in this country. To prevent any confusion arising from its being mistaken for the thermometer first described, which is called from its original maker, the Fahrenheit thermometer, the letter F. is placed after temperatures indicated by his thermometer, and the letter C. after those denoted by the centigrade.

From its zero-point each thermometer counts downwards as well as upwards; and to distinguish the degrees below zero from those above it, the former are distinguished by prefixing to them the minus sign -. Thus, is said to freeze at mercury 37.9° F.; that is, at — 37.9° below Fahrenheit's zero.