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Physical Properties of Gaseous Water
Physical Properties of Gaseous Water related with the boilingpoint of a liquid is defined as the highest temperature attainable by a liquid under a given pressure of its own vapour when heat is applied externally and evaporation occurs freely from the surface. Under a normal pressure of 760 mm. of mercury, water boils at 100° C., and the boilingpoints at various other pressures are given as follows:
Variation of the Boilingpoint of water with Pressure
The hypsometer is a small, portable piece of apparatus which enables the boilingpoint of water to be determined at any place. The water is placed in a small tube or boiler and is heated by means of a spirit flame beneath, whilst the vapour in its passage to the open air heats a delicate thermometer. The instrument is sometimes used for determining the altitude of a place, since the boilingpoint of water falls through one degree C. for every 1080 feet rise above sea level. A more general expression is that of Soret, namely: h = 295(100t) where h is the height above sea level expressed in metres, and t the ebullition temperature. A knowledge of the variation of the vapour pressure in the neighbourhood of 100° is frequently of value in checking the accuracy of thermometers by immersion in steam at atmospheric pressure. In this connection, therefore, the following data are of interest:
Liquid water is incapable of existence above 374° C., this being the critical temperature, the corresponding pressure being 200 atmospheres and the volume 0.00386 approximately. The density, d, of saturated steam at various temperatures is given by the equation: d = 0.4552 – 0.0004757(t  160) – 0.000000685 (t  160)^{2} in grams per c.c. where t is the temperature on the centigrade scale. Physical Properties of Gaseous Water arrount critical temperature. Assuming the critical temperature to be 365° C., the critical density becomes 0.329 gram per c.c. The more recent work of Holborn and Baumann, however, suggests that 374.3° is a closer approximation than 365° to the critical temperature, and if this value is inserted for t in the above equation, the figure for d_{c} becomes 0.322. This is probably the most accurate value. The viscosity of water vapour at 20° C. is 0.0000975. The latent heat of vaporisation of water at 100° C. is 539 calories 15°. Sometimes the value is given for water at 0° C., in which case the amount of heat required to raise the water from 0° to 100° C. must be added to the above quantity. The following are the most noteworthy attempts to determine the latent heat of steam, undoubtedly the most accurate results being those of Richards and of Mathews. It is remarkable that the values obtained by Black and by Watt should approximate so closely to that accepted at the present day. The amount of heat required to raise 1 gram of water at 0° C. into vapour at t° C. is given in calories 15° by the expression: 639.11 + 0.3745(t100) – 0.000,990(t  100)^{2}. The mean specific heat of steam at constant pressure between 100° and 1400° C. is given by the equation: Cp = 0.4669 – 0.000,016,8t + 0.000,000,044t^{2}, the experimental data being as follows: Molecular specific heat of steam constant pressure
The molecular specific heat at constant volume is given by the expression: Cv = 5.91 + 0.003,76t – 0.000,000,1552t^{2}; between 0° and 2900° by: Cv = 5.750 + 0.783×10^{3}T + 0.626×10^{6}T^{2} + 4.56×10^{10}T^{3} – 2.18×10^{17}T^{5}, where T is the absolute temperature, and Cv = 6.065 + 0.0005t + 0.2×10^{9}t^{3} between 1300° and 2500° C. The ratio CJCV has been evaluated at 1.29, agreeing fairly well with the figure expected of a substance consisting of triatomic molecules. The value falls with rise of temperature as is usual. Thus:
Watervapour exerts a distinct selective action on light, the effect of atmospheric moisture being detectable spectroscopically in sunlight. It manifests absorption in the infrared region, and the "a" absorption band is interesting as being the one by which the presence of water vapour on Mars was first determined by Slipher in 1908. 
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