They are not so well adapted for travellers, nor for measurements of considerable elevations, as aneroids.
CHAPTER VI.
INSTRUMENTS FOR ASCERTAINING TEMPERATURE.
53. Temperature is the energy with which heat affects our sensation of feeling.
Bodies are said to possess the same temperature, when the amounts of heat which they respectively contain act outwardly with the same intensity of transfer or absorption, producing in the one case the sensation of warmth, in the other that of coldness. Instruments used for the determination and estimation of temperatures are called Thermometers.
Experience proves that the same body always occupies the same space at the same temperature; and that for every increase or decrease of its temperature, it undergoes a definite dilatation or contraction of its volume. Provided, then, a body suffers no loss of substance or peculiar change of its constituent elements or atoms, while manifesting changes of temperature it will likewise exhibit alterations in volume; the latter may, therefore, be taken as exponents of the former. The expansion and contraction of bodies are adopted as arbitrary measures of changes of temperature; and any substance will serve for a thermometer in which these changes of volume are sensible, and can be rendered measureable.
54. Thermometric Substances.—Thermometers for meteorological and domestic purposes are constructed with liquids, and generally either mercury or alcohol, because their alterations of volume for the same change of temperature are greater than those of solids; while being more manageable, they are preferred to gases. Mercury is of all substances the best adapted for thermometric purposes, as it maintains the liquid state through a great alteration of heat, has a more equable co-efficient of expansion than any other fluid, and is peculiarly sensitive to changes of temperature. The temperature of solidification of mercury, according to Fahrenheit’s scale of temperature, is -40°; and its temperature of ebullition is about 600°. Sulphuric ether, nitric acid, oil of sassafras, and other limpid fluids, have been employed for thermometers.
55. Description of the Thermometer.—The ordinary thermometer consists of a glass tube of very fine bore, having a bulb of thin glass at one extremity, and closed at the other. The bulb and part of the tube contains mercury; the rest of the tube is a vacuum, and affords space for the expansion of the liquid. This arrangement renders very perceptible the alterations in volume of the mercury due to changes of temperature. It is true, the glass expands and contracts also; but only by about one-twentieth of the extent of the mercury. Regarding the bulb, then, as unalterable in size, all the changes in the bulk of the fluid must take place in the tube, and be exhibited by the expansion and contraction of the column, which variations are made to measure changes of temperature.