Page:Elementary Text-book of Physics (Anthony, 1897).djvu/235

[§ 207 Chlorine, sulphur dioxide, ammonia, nitrous oxide, carbon dioxide, and several other gases, become liquid under sufficient pressure. Andrews found that at a temperature of 30.92° pressure ceases to liquefy carbon dioxide. This is the critical temperature for that substance. The critical temperatures of oxygen, hydrogen, and the other so-called permanent gases, are so low that it is only by methods capable of yielding an extremely low temperature that they can be liquefied. By the use of such methods any of the gases may be made to assume the liquid state. In the case of hydrogen, however, the low temperature necessary for its liquefaction has only been reached by allowing the gas to expand from a condition of great condensation, in which it had already been cooled to a very low point. The first successful attempts to condense these gases were made by Cailletet and Pictet, working independently. The best work on the subject has been done by Olszewski, who has succeeded in obtaining large quantities of liquid oxygen, nitrogen, and hydrogen, and in freezing nitrogen.

209. Pressure and Density of Saturated Gases and Vapors.—It has been seen that, for each gas or vapor at a temperature below the critical temperature, there is a maximum pressure which it can exert at that temperature. To each temperature there corresponds a maximum pressure, which is higher as the temperature is higher. A gas or vapor in contact with its liquid in a closed space will exert its maximum pressure.

The relation between the temperature and the corresponding maximum pressure of a vapor is a very important one, and has been the subject of many investigations. The vapor of water has been especially studied, the most extensive and accurate experiments being those of Regnault.

210. Pressure and Density of Non-saturated Gases and Vapors.—If a gas or vapor in the non-saturated condition be maintained at constant temperature, it follows very nearly Boyle's law (§ 105). If its temperature be below its critical temperature, the product of volume by pressure diminishes, and near the point of saturation the departure from the law may be considerable. At this point