compressing the air in the bulb B. The inverse takes place if B be warmed while A is kept cool. Or if both bulbs be warmed,

but one more than the other, the air most warmed will expand most, and the difference of temperature will be shown by the depression of the liquid in the stem of the bulb which is most warmed. If both bulbs be equally warmed, no motion of the liquid takes place, because equal forces of expansion act in opposite directions. Such a differential thermometer may be used, for instance, to compare the heat of the hand of a sick person, with that of the hand of one in health.

§ 49. Air-thermometers may also be used as pyrometers in measuring the heat of furnaces. For this purpose (fig. 19) a flask of platinum, P (a difficultly fusible metal), is fastened to a very narrow glass tube, T, open at A. Both flask and tube being full of air,

A

Fig. 19.

T

P

the former is placed in the furnace F, whose temperature is to be measured. The air in P expands, and the excess escapes by A. When the flask P has got as hot as the furnace can make it, the end A is closed by the blowpipe, and the flask is removed from the furnace. When cold, the end of A is broken off under mercury, and the quantity of mercury which thereupon enters the tube T and the flask P, measures the temperature to which the latter has been exposed.

§ 50. The expansion of solids is also used for the measurement of very high temperatures-that is, for pyrometry. "Daniell's

pyrometer " consists of a bar of platinum standing in a vertical position in a hole in a block of earthenware. The top of the platinum bar touches a plug of earthenware which rests upon the platinum and projects out of the hole. The plug is made to move with difficulty. The platinum, on expanding in the furnace, pushes up the plug of earthenware, which remains raised when the pyrometer is removed from the furnace. The degree to which the little plug has been pushed up is exactly measured (by applying it to the shorter arm of a lever), and gives comparatively the heat of the furnace.

§51. Liquid thermometers are those most often used for the measurement of all temperatures not exceedingly high. Two liquids are almost the only ones used in thermometers, namely, spirits of wine and mercury. Spirits of wine or alcohol cannot be frozen, and is therefore used to measure exceedingly low temperatures; but it easily boils, and is therefore useless for high ones. Mercury, on the other hand, can be frozen by severe cold; but it boils only at a very high temperature. Mercury, moreover, has the great advantage of great cohesion to itself, and little adhesion to glass; hence its surface is always sharply defined, and the mass of mercury unbroken. It will be sufficient to describe the construction of the mercurial thermometer.

§52. Mercurial thermometer.—A glass tube is formed having an exceedingly fine (capillary) bore of uniform capacity throughout. The shape of the bore in section is advantageously that of a slit; and its uniformity is tested by seeing whether a little mercury introduced into it always occupies the same length, to whatever part of the tube it is moved. Upon one end of such a tube, a cup, C,, is blown, and upon the other a bulb, B,, of less capacity than the cup (fig. 20, 1). More pure dry mercury is poured into the cup C, than is sufficient to fill the bulb B. The mercury does not descend into B, on account of the air in B1, and because of the narrowness of the tube. This stage is seen in 1. The bulb B, is warmed, the air in it expands, and some of it escapes and bubbles through the mercury in C1. On cooling B, some mercury follows the shrinking air, and drops into B,. Heat is

C

then applied to B, sufficient to boil the mercury in it. The vapour of mercury drives before it all the air in the bulb and stem, and

3

forces it through the mercury in C1. On withdrawing the flame from B,, the vapour of mercury condenses; and the atmospheric pressure forces the mercury in C, down the stem, until the bulb B, is completly filled. This is shown in 2. The cup C2 is then cut off, as in 3. The bulb B, is then again heated as high as the temperature to which the thermometer has subsequently to be exposed. The excess of mercury as it expands, drops off from the end of the tube. When no more is expelled, some hot wax, W, is dropped upon the open end, so as to close it completely, 4. The mercury now on cooling shrinks; and since no air can enter, the bore of the stem above the mercury is a vacuum. Finally, the end of the tube below the wax is softened and closed; this the pressure of the air upon the sides of the empty stem assists.

§ 53. To graduate the thermometer, it is necessary that the height of the mercury, at at least two fixed and constant temperatures, should be known. For reasons which will be explained in §§ 85-96 under latent heat, the temperature of melting ice is constant, and is the same as that of the water which it forms on melting. Hence, if coarsely pounded ice be allowed to melt, it

will remain continually of the same temperature as long as any ice remains unmelted. Further, for a similar reason (also described under latent heat, § 85) boiling water has always the same temperature, and the same as that of the steam which is formed from it *.

§ 54. The thermometer (5, fig. 20) is plunged into melting ice, and the point to which the mercury sinks in the stem is marked upon the glass. It is then plunged into boiling water, and the point to which the mercury rises is again marked. We know now that whenever the mercury stands at the lower point, its temperature is that of melting ice; and whenever at the higher, it is that of boiling water. When it stands between the two, it is of intermediate temperature. The glass stem between the two fixed points is divided into equal parts, called degrees. In different countries the number of these divisions is different. There are three principal systems of division. The simplest, and that most frequently used for scientific purposes, is the Centigrade or Celsius. On the Centigrade thermometer the temperature of melting ice is called zero (marked 0°C.); the temperature of boiling water is 100 (marked 100° C.). The intervening space is divided into 100 equal divisions or degrees. In Réaumur's thermometer, which is in popular use in Germany and Russia, the temperature of melting ice is 0°, that of boiling water is 80 (marked 80° R.), and there are 80 equal divisions or degrees between the two. On the Fahrenheit scale, the melting-point of ice is marked 32° F., and the boiling-point of water 212° F.; there are therefore 180 degrees between them. The Fahrenheit thermometer is in popular use in England.

?

* The influence of pressure not considered. How this affects the temperature at which a liquid boils, will be seen under Ebullition, § 138.

CHAPTER V.

RELATION BETWEEN THE THERMOMETRIC SCALES IN COMMON USE.

$55. The relation between the Centigrade, Réaumur, and Fahrenheit scales is seen in fig. 21, where the three thermo

meters are supposed to be of exactly the same size.

C. graduated according to the Centigrade scale.

It is clear that the length of a degree on the C. scale is to the length on the R. scale inversely as 100 is to 80, or directly as 4 is to 5. The length of a degree C. is to the length of a degree F. as 180 to 100, or as 9 to 5; and the length of a degree R. is to the length of a degree F. as 9 to 4.

If, therefore, the mercury stands at a certain number of degrees C., and we wish to know what it is on the R. scale, we must take a less number of the larger R. degrees, and indeed a number bearing the inverse proportion to the number C. that the size of the R. degree bears to the size of the C. degree; or