MOLECULAR WEIGHTS.
It can be shown. further, that the greatest possible value of 7, the ratio of the specific heats, is 1.67; but if the molecules are complex, so that there is internal energy in them, 7 must be less. It is interesting to note that for helium, argon, and mercury vapor 7 =1.67, as found by direct experiment.
The properties of the pressure due to the at mosphere around the earth are discussed in the article ATMOSPHERE. Only a few points need be mentioned here. The pressure is measured by a barometer (q.v.) and is found to nearly equal that of 76 centimeters of mercury at sea-level and at 45° latitude, i.e. 76 X 13.6 X 980 or 1,013,300 dynes per square centimeter. The barometer was invented by Torricelli, a pupil of Galileo; and the first instrument was made and used by Viviani in 1643. Pascal in 1648 showed that the height of the barometer varied with different heights above the earth, and proved that the pressure of the atmosphere obeyed the laws of liquid pressure.
Von Guericke invented the air-pump (q.v.) in 1650, and without knowing of Torricelli's work discovered the properties of atmospheric pres sure. He did not publish an account of his work, however, until 1672. Boyle published in 1660 an account of his experiments with an air pump illustrating the properties of the pressure due to the air.
The action of lift-pumps, siphons, etc., depends upon atmospheric pressure. Air-pumps are in struments designed to exhaust the gas from a closed space such as a glass bulb.
Consult: Kimball, Physical Properties of Gases (Boston, 1890) ; Barus, Laws of Gases, "Scientific Memoir Series." vol. v. (New York, 1899) ; Ran dall, Expansion of Gases, "Scientific Memoir Series," vol. xv. (New York, 1901) ; Tait, Proper ties of Matter (Edinburgh, 1885) ; Meyer, The Kinetic Theory of Gases (London, 1899). See DIFFUSION; EFFUSION; MATTER, THEORIES OF; AVOGADRO'S RULE; CHEMISTRY; SOLUTION. See al so VAPOR.