A short manual of inorganic chemistry / by A. Dupré and H. Wilson Hake.

  • August Dupré
Date:
1892
Catalogue details

Licence: Public Domain Mark

Credit: A short manual of inorganic chemistry / by A. Dupré and H. Wilson Hake. Source: Wellcome Collection.

    50/424 page 30
    marked. These two fixed points, respectively known as the freezing point and the boiling point, give us a standard interval which serves as a basis for a scale of degrees. For measuring moderate or very low temperatures an alcohol thermo- meter is used. It must be graduated by comparison with a mercury thermometer. As the rate of expansion of mercury increases with the temperature, the indications of the mercury thermometer are not absolutely correct at higher temperatures; hence, where complete accuracy is required (and also for high temperatures), an air thermometer must be used, and this may be provided with a platinum bulb. The indications of this instrument are perfectly accurate, but involve with each observation a somewhat complicated calculation, depending upon the fact that, as the change of volume of any given bulk of air, for any known change of temperature (and pressure) can be calculated, it follows that the temperature can be calculated, if the change of volume is known (p. 51). The following are the ranges of temperature, for which these dif- ferent kinds of thermometers are available:—Mercury thermometer, from — 38° to 360°; alcohol thermometer, from — 100^ to 40Q; while an air thermometer is practicably available for any known temperature. Very high temperatures may be estimated by immersing in the substance whose temperature has to be determined, a known weight of a substance, which does not melt at that temperature (generally platinum), and cooling it in a known weight of water. The rise of temperature being observed, and the specific heat of platinum known, the required temperature may be calculated (p. 31). Or the variation of the conducting power of platinum for electricity, which diminishes as the temperature rises, may be employed. Conversion of Degrees of Different Scales.—There are three scales in common use—that of Fahrenheit, used in England, that of Celsius. (Centigrade), used in France, and that of Reaumur, used in Germany These are marked as to their freezing points respectively, 32^,* 0°, and 0°; and, as to their boiling points, 212°, 100°, and 80°. Hence, the standard interval in the Fahrenheit is divided into 180°, in the Celsius into 100°, and in the Reaumur into 80°; hence, the relation between their degrees is as 9 : 5 : 4. *32° Fahrenheit.—A mixture of ice and salt (frequently used as a freezing mixture) attains a temperature of 32° Fahrenheit below the freezing point of water, and Fahrenheit believed this to be the lowest possible temperate, hence he took it for bis zero. We now know that the real zero is far lower (see Absolute Zero, p. 39).
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