=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 deg.; and its temperature of ebullition is about 600 deg. 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. 56. STANDARD THERMOMETER. The peculiarities in the construction of thermometers will be best understood by describing the manufacture of a _Standard Thermometer_, which is one of the most accurate make, and the scale of which is divided independently of any comparison with another thermometer. Fig. 36 is an illustration of such an instrument, on a silvered brass scale. [Illustration: Fig. 36]