in bringing the walls of a building back to their original position after they had begun to separate from each other.

Sensible and Latent Heat.

Temperature.

180. When the heat received by a body is attended with an increase of warmth, the heat is said to be sensible.

In certain cases a body may receive a large amount of heat without any increase of warmth. Such heat is said to be latent.

The temperature of a body is the degree of heat which it manifests: if the body grows warm, its temperature is said to rise; if it grows cool, its temperature is said to fall.

II. THERMOMETERS.

The Thermometer.

181. A THERMOMETER is an instrument for measuring temperatures.

The thermometer depends upon the principle that bodies expand when heated, and contract when cooled. Thermometers have been constructed of a great variety of materials. For common purposes, the mercurial thermome ter is preferred, on account of the uniformity with which both mercury and glass expand when heated.

The mercurial thermometer consists of a bulb of glass, at the upper extremity of which is a narrow tube of uniform bore, hermetically sealed at its upper end. The bulb and a part of the tube are filled with mercury, and the whole is attached to a frame on which is a scale for measuring the rise and fall of the mercury in the tube.

(180.) What is sensible heat? Latent heat? Temperature? (181.) What is a Thermometer? On what principle does it depend? What is the best thermometer for common use? Describo a mercurial thermometer.

Method of making a Thermometer.

182. A capillary tube of glass is provided, of uniform bore, upon one end of which a bulb is blown, and upon the other a funnel, as shown in Fig. 130.

The funnel is nearly filled with mercury, which is at first prevented from penetrating into the bulb by the resistance of the air and the smallness of the tube. The bulb is therefore heated, when the air within expands, and a portion escapes in bubbles through the mercury. On cooling, the pressure of the external atmosphere forces a quantity of mercury through the tube into the bulb. By repeating this operation a few times, the bulb and a portion of the tube are filled with mercury.

The whole is then heated till the mercury boils, thus filling the tube, when the funnel is melted off and the tube hermetically sealed by means of a jet of flame urged by a blow-pipe. On cooling, the mercury descends to some point of the tube, as shown in Fig. 131, leaving a vacuum at the upper point. It only remains to graduate it, and attach a suitable scale.

Method of Graduation.

183. Two points of the stem are first determined, the freezing and the boiling point. These are determined on the principle that the temperatures at which distilled water freezes and boils, are always the same, that is, when these changes of state take place under equal atmospheric pressures.

The instrument is first plunged into a bath of melting ice, as shown in Fig. 132, and is allowed to remain until it takes the

(182.) Describe the process of filling a thermometer with mercury. How is the tube sealed? (183.) On what principle are the freezing and boiling points determined?

temperature of the mixture, say twenty or thirty minutes. A slight scratch is then made on the stem at the upper surface of the mercury, and this constitutes the

freezing point.

The instrument is next plunged into a bath of distilled water, in a state of ebullition, care being taken to surround it with steam by means of an apparatus like that shown in Fig. 133. After the mercury ceases to rise in the tube, which will be in a few minutes, the level of its upper surface is marked on the stem, by a scratch, as before, and this constitutes the boiling point.

The space between the boiling and freezing points is then divided into a certain number of equal parts, and the graduation is continued above and below as far as may be desired. These divisions may be scratched upon the glass with a diamond, or, as is usually done,

t

they may be made on a strip

of metal, which is attached

Fig. 132.

to the frame. The divisions are numbered according to the kind

of scale adopted.

Thermometer Scales.

184. Three principal scales are used: the Centigrade scale, in which the space between the freezing and boiling

How is the freezing point determined? The boiling point? How is the intermediate space divided? (184.) What are the three principal scales used?

points is divided into 100 equal parts, called degrees; Reaumur's scale, in which the same space is divided into 80 equal parts, called degrees;

and Fahrenheit's scale, in which this space is divided into 180 equal parts, also called degrees.

In the centigrade scale, the freezing point is marked 0, and the degrees are numbered both up and down, the former numbers being. considered positive, and designated by the sign +, whilst the latter are considered negative, and designated by the sign Of course the boiling point is marked 100°.

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Fig. 134 represents a thermometer mounted and graduated according to the centigrade scale. In it the mercury indicates 30° C.

In Reaumur's scale, the freezing point is marked 0, and the boiling point 80°. The degrees below freezing are marked as in the centigrade scale.

Fig. 188.

anotary

In Fahrenheit's scale, which is the one most used in the United States, the 0 point is taken 32° below the freezing

Where is the 0 point of the centigrade scale? Explain the signs + and -. What is the boiling point marked? Where is the 0 of the Reaumur scale? The boiling point? Where is the Q of Fahrenheit's scale?

1

point, and the divisions are numbered from this point both up and down. The boiling point of distilled water is, then, 212° F.

Conversion of Centigrade

and Reaumur's Degrees into Fahrenheit's.

185. A degree on the centigrade scale is equal to one and eight tenths of a degree on the Fahrenheit scale, and one on Reaumur's scale is equal to two and a quarter on Fahrenheit's. Hence, to convert the reading on a centigrade to an equivalent one on Fahrenheit's scale, multiply it by 1.8 and add to the result 32°. Thus, a reading of 25° centigrade, is equivalent to 25° × 1.8 + 32°, or 77° F. To convert a reading on Reaumur's scale to an equivalent one on Fahrenheit's, multiply by 24, and to the result add 32°. Thus, a reading of 24° Reaumur is equivalent to 24° x 2 + 32°, or 86° F.

By reversing the above processes, readings on Fahrenheit's scale may be converted into equivalent ones on the centigrade or Reaumur's -scale.