CHAPTER V. HEAT. LESSON XX.-TEMPERATURE. 163. It is proposed in the present chapter to discuss that form of molecular energy which we term heat. As, however, this word is used to denote two kinds of energy-one of which is capable of residing in a body, while the other kind traverses space with an enormous velocity—we shall at present confine our attention to the first of these, leaving the last, or radiant energy, to form the subject of another chapter. It will be convenient to consider, in the first place, the various effects of heat upon matter; secondly, the laws regulating the distribution of heat through space; and thirdly, the relation between heat and other kinds of energy. 164. Temperature. In the first place, let us consider temperature. This word is used to denote the state of a body with respect to sensible heat. To illustrate the meaning of the word, let us suppose that two substances, such as a quantity of water and of mercury, each contain heat to such an extent that when they are brought intimately together there is no transference of heat from the one to the other, but each keeps what it has; then these two substances are said to be of the same temperature. But if, when the water and the mercury are shaken together, the water parts with some of its heat to the mercury, then the water is said to be of a higher temperature than the mercury; while, on the other hand, if it receives heat from the mercury, it is said to be of a lower temperature than the mercury. 165. Bodies in general expand through Heat.-Let us take a brass ball and a ring, such that at the ordinary temperature of the air the ball will pass through the ring. Now if we heat the ball intensely by a flame, owing to the expansion occasioned by heat we shall no longer be able to force it through the ring. Next let us take a bladder that is nearly but not quite filled with air, and place it beside the fire; the air within the bladder will soon expand through the heat, so that the bladder will appear to be quite full of air. There are, however, exceptions to the law of expansion. Thus water between the temperatures of o° C. and 4° C. contracts instead of expanding through an increase of heat, while after 4° C. it begins to expand in the usual way, at first very slowly, but as the temperature rises, with increasing rapidity. 166. Measurement of Temperature by Thermometers.— As we can only perceive heat through its effects upon bodies, we must make use of some one of these as a means of measuring it. The expansion caused by heat is probably the effect most convenient for this purpose; and if the same body always expanded to the same extent for equal increments of temperature, there would be no difficulty in measuring temperature exactly by this means; but this is ar from being the case. Thus a gramme of water will occupy the same volume at o° C. and at 8° C., so that in this case we cannot correctly estimate the temperature by means of the volume. In fact water near its freezing-point (0° C.) is undergoing very rapid molecular changes, and in general liquids near their freezing-points, or solids near their melting-points, are not well fitted to be used as the means of measuring temperature by their change of volume. On the other hand, a gas such as atmospheric air, which is incapable of being condensed into a liquid by the most extreme cold, is admirably adapted for the purpose of measuring temperature as far as accuracy is concerned. Nevertheless, an air ther mometer is a most inconvenient instrument for ordinary use. A mercurial thermometer is best adapted for general purposes, being very convenient and tolerably accurate, although when extreme accuracy is desired it ought to be corrected by means of an air thermometer. 167. Mercurial Thermometer. This instrument is constructed on the principle that mercury expands more than glass, In order to make a mercurial thermometer, let us take a glass tube, having a narrow capillary bore with a bulb blown at one end of it, the other end being in the meantime open so that the bulb is filled with air. Let the bulb next be heated over a lamp, in consequence of which the air in the bulb will expand, and part will be driven out at the mouth of the tube. Next, before the bulb begins to cool, let the mouth of the tube be plunged beneath the surface of a vessel filled with pure mercury. During the process of cooling, the air left in the bulb will contract, and the pressure of the atmosphere will cause the mercury to rise in the tube until part of it gets into the bulb. Having by this means got some mercury into the bulb, the next operation is to boil the mercury in the bulb until not only the bulb but also the capillary tube is filled with the vapour of mercury. When this has been accomplished, let the end of the tube be once more plunged into the basin of mercury. As there is now no air in the tube or in the bulb, but only vapour of mercury, when this cools there will be a vacuum, and the mercury into which the instrument is plunged will be driven up by the atmospheric pressure until it fills the bulb. When the bulb and tube have by this process been filled with mercury, the tube is then hermetically sealed; and when the instrument has cooled, it will be found that the mercury will fill the bulb and part of the tube, the other part being left empty. If we heat an instrument of this kind, the glass of the bulb will expand through heat, and likewise the mercury; but the mercury will expand more than the glass, and the consequence will be that the mercurial column will rise in the stem. In like manner, if the instrument be cooled, the mercury will contract more than the glass, and the column of mercury will fall. If the capillary bore be fine enough, a large rise of the column of mercury may be caused by a comparatively small elevation of temperature, and a thermometer may thus be made to indicate differences of temperature with very great delicacy. 168. Determination of Fixed Points.-Having thus constructed our instrument, the next operation is to mark off on the stem the heights of the mercurial column corresponding to the freezing and the boiling-points of water. To ascertain the freezing-point, the instrument is plunged into some melting ice, where it is allowed to remain for about a quarter of an hour. A mark is then scratched on the stem at the termination of the mercurial column. This point denotes zero of the centigrade scale. The next thing is to determine the boiling-point of water, and here it must be borne in mind that this point is not constant like the freezing-point, but varies with the pressure of the atmosphere; indeed it is well known that water will boil at a much lower temperature in an exhausted receiver than in the open air. Let us suppose, for the sake of simplicity, that this pressure at the moment when the experiment is made is exactly equal to that of a column of mercury 760 millimetres in length, and having the temperature of the freezing-point of water; in other words, let the barometric height be 760 millimetres. Let us now immerse the thermometer in steam arising from pure water boiling under this pressure, and mark off as before the termination of the mercurial column. The point so marked will denote 100° on the centigrade scale. In marking off this point it is necessary that not only the bulb but also the stem of the thermometer up to the very point marked should be exposed to the steam, and in order to do this properly we make use of an instrument which is represented both externally and internally in Fig. 53. The thermometer tube is inserted into a thick piece of indiarubber, which is made to cover tightly the top of the instru-、 ment, and the stem is lowered until the mercurial column just appears above this cover when the water is boiling; all the stem is thus exposed to the action of the steam. The bulb of the thermometer is not plunged into the water, but remains suspended above it; and the steam is conveyed first up through an interior chamber, and then down again, until it finally leaves the exterior vessel through the aperture The whole of the thermometer is thus well surrounded C. by the steam, and by a cylinder which has the temperature of the steam. 169. Graduation. Having thus ascertained and marked off the two fixed points, the next thing is to graduate the instrument. If the bore of the capillary tube be of equal size throughout, the divisions denoting degrees will all be of equal length; and if the centigrade scale be adopted, there will be just one hundred of these spaces between the freezing M |