Since the 1600s, chemists have known that temperature can determine whether a substance exists as a gas, liquid, or solid. Johannes van Helmont, who coined the term gas to describe carbon dioxide, used a different term, vapors, to describe substances that became gaseous only when heated, such as water. During the late 1700s, scientists learned that when a gas is cooled, its volume is reduced by a predictable amount. Cooling slows down the motion of the gas molecules, so they take less space. Similarly, pressurizing a gas, or forcibly squeezing its molecules closer together, reduces its volume. Eventually, through cooling and compression, the volume of a gas can be reduced by so much that its molecules collapse upon each other and come into contact--in other words, the gas changes into a liquid. Compression and cooling soon became the twin tools of scientists attempting to liquefy gases.
The first scientist to liquefy a substance that normally exists as a gas was Gaspard Monge, a French mathematician who produced liquid sulfur dioxide in 1784. But most gases were not liquefied until the mid-1800s, beginning in 1823 when Michael Faraday liquefied chlorine. Faraday pressurized chlorine gas inside a curved glass tube that was submerged at one end in a beaker of crushed ice. Under pressure, the gas changed into liquid chlorine when cooled by the ice near the end of the tube. Faraday also liquefied carbon dioxide, hydrogen sulfide, and hydrogen bromide in a similar manner. More than twenty years later, after pursuing other research, Faraday returned to gas liquefaction. By then, more effective cooling agents had been developed. But despite the combined effects of cooling and compression, Faraday was unable to liquefy several gases, such as oxygen and hydrogen, which he called " permanent" gases. Then in the late 1840s, Irish physical chemist Thomas Andrews (1813-1885) suggested that every gas has a precise temperature--which he called the critical temperature --above which the gas cannot be liquefied even under greater pressure. Andrews reached his conclusion by observing the behavior of pressurized liquid carbon dioxide. Andrews, who earned his medical degree in Scotland after studying at one of France's leading research laboratories, spent his entire scientific career in Belfast, Ireland, where he practiced medicine while teaching chemistry and pursuing his research interests.
Andrews' concept of critical temperature soon led to a breakthrough in the liquefaction of the so-called permanent gases. To reach temperatures low enough to liquefy these gases, two scientists independently came up with the idea of using a "cascade" process, which reduces temperature step by step. In this method, one liquefied gas is used to cool a second gas that has a lower critical temperature; then the second gas, when liquefied, is used to cool another gas with an even lower critical temperature; and so on.
In 1877, French physicist Louis Paul Cailletet (1832-1913) succeeded in liquefying three permanent gases--oxygen, nitrogen, and carbon monoxide --via the cascade process. Cailletet had begun his career working at his father's iron foundry and doing metallurgical research, but he soon became interested in studying the behavior of gases. In addition to his gas liquefaction work, Cailletet built high-pressure manometers and an altimeter (used today to measure the height at which an airplane flies).
Around the same time, Swiss chemist Raoul Pierre Pictet (1846-1929) liquefied oxygen using methods very similar to Cailletet 's, and the question of who had done it first was vigorously debated, although Cailletet demonstrated priority. Because Pictet' s equipment was more elaborate, he was able to produce greater quantities of liquid oxygen. Pictet's interest in gas liquefaction arose from his attempts to produce ice artificially for use as a refrigerant. He began his career as a physics professor in Geneva, then continued his research work in Berlin and Paris.
Although most gases had been liquefied by the late 1800s, commercial production of them was unfeasible. German chemist Karl von Linde (1842-1934) changed that in 1895, when he invented a continuous process for producing large quantities of liquid air (mostly nitrogen and oxygen). British chemical engineer William Hampson (1859-1926) invented a similar liquefaction method around the same time. Von Linde became an engineering professor in Munich, where he became interested in low-temperature research. In 1876 he developed the first practical refrigerator.
Von Linde's commercial liquefaction process makes use of a phenomenon called the Joule-Thomson effect, named for its discoverers, James Joule and Lord Kelvin ( William Thomson). These scientists had shown in 1853 that a compressed gas becomes cooler when it expands, assuming it does not absorb heat from its surroundings. In von Linde's process, which is still the basis for modern practice, liquid air is cooled and compressed, then allowed to expand, which cools it even more. The cold air is constantly recycled to cool more incoming compressed air. Because of the cumulative cooling effect, the air gradually becomes cold enough to liquefy. Von Linde's process immediately became a commercial success and laid the foundation for today's liquid air production industry.
Von Linde also developed more economical methods of separating the liquid oxygen and liquid nitrogen, which both found many practical uses in research and industry. Incandescent light bulbs filled with nitrogen, for example, lasted longer than earlier bulbs, which used a vacuum instead of a filler gas. Liquid nitrogen is also quite useful for instantaneous freezing. In biological research, liquid nitrogen is used to freeze blood cells, sperm, tissues, and even whole small organisms. When frozen, the cells stop normal activities, allowing scientists to examine a "freeze-frame" of cellular life.
Meanwhile, hydrogen gas had stubbornly resisted all scientific attempts to liquefy it until 1898, when Scottish chemist and physicist Sir James Dewar applied the Linde process in larger, more efficient equipment. Dewar used liquid air to pre-cool compressed hydrogen, which reduced its temperature enough to liquefy the gas by expansion. (Some gases such as hydrogen have a very low inversion temperature; if the gas is expanded above this temperature, it becomes warmer instead of cooler.) A year later, in 1899, Dewar succeeded in solidifying hydrogen.
Dewar, a professor at universities in London and Cambridge, had become interested in the field of extremely low temperatures during the 1870s, when permanent gases first began to be liquified. In 1891, Dewar produced liquid oxygen in large quantities and studied its magnetic properties. The following year, he invented the Dewar flask --the ancestor of today's familiar ThermosTM bottle--which maintains the temperature of liquids by insulating them. Dewar's silver-coated glass flasks were double-walled, with a vacuum between the walls.
Near the end of the century, a new family of elements called the inert, or nonreactive, gases was discovered by Sir William Ramsay and his co-workers. These gases, which include argon, helium, and neon, presented another challenge to scientists interested in liquefaction. Dewar came very close to liquefying helium, but his sample of the gas also contained neon, which froze and blocked the valves on his equipment. A few years later, in 1908, Heike Kamerlingh Onnes succeeded in producing liquid helium using techniques similar to Dewar's. The compressed helium gas was pre-cooled by liquid hydrogen before undergoing expansion cooling, as in the Linde process. Kamerlingh Onnes' equipment was rather elaborate; later, other scientists developed simpler helium liquefiers that could produce greater amounts of the liquid gas.
Other research related to gas liquefaction was concerned with temperature measurement. British chemist Morris William Travers (1872-1961) was the first scientist to measure the temperature of liquid gases accurately. After working in India at a newly-founded science institute, Travers returned to England and became director of a glass manufacturing plant, which led to his early interest in furnaces and fuel technologies, such as coal gasification. In 1894, Travers began working with Ramsay on isolating the new inert gases. He shares credit with Ramsay for discovering krypton in 1898. Independently of Dewar, Travers constructed equipment for liquefying hydrogen, which he and Ramsay used to obtain neon from air. Late in life, Travers wrote a biography of Ramsay that recounted their exciting work on the inert gases.
Today key commercial uses of liquified gas include liquified refridgerants for cryogenic application and liquified petroleum gases for fuel use.
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