Cryogenics 1

It is hard to know when Dewar first resolved to make the study of matter at low temperatures his principal field of research. He had spoken on the latent heat of liquefied gases at the meeting of the British Association in 1874, but his paper was not printed so we do not know what he said. ² He was a keen student of the history of chemistry, going so far one year as to choose alchemy as the subject of the customary course of six lectures ‘for a juvenile auditory’ that was offered, after each Christmas, in the Royal Institution. When he accepted the Fullerian chair there in April 1877 he knew much of Davy and Faraday's work on the liquefaction of such gases as chlorine, ammonia, nitrous oxide and hydrogen sulphide. Faraday had been the first to liquefy six such gases. He and his contemporaries had realised that although many gases could be liquefied by pressure at temperatures accessible with solid carbon dioxide, others required cooling to temperatures well below this before liquefaction could be induced by compression, however great. The distinction between the two classes was finally made clear by Thomas Andrews in experiments at Belfast in the 1860s and set out in his Bakerian Lecture ³ to the Royal Society in 1869. For each substance there is a critical temperature above which the liquid state, in equilibrium with its vapour, cannot exist (Figure A.1). ⁴ If, for any substance, this critical temperature is above about 200 °C then that substance is usually already a liquid at atmospheric temperature and pressure. If it is lower, then the substance is usually a gas but it may be readily condensable. Thus ammonia, with a critical temperature of 133 °C, is condensed at room temperature (say, 20 °C) by a pressure of seven atmospheres or more. Carbon dioxide, with a critical temperature of 31 °C, condenses at room temperature only if the pressure is above about 50 atmospheres. Ethylene (T _c= 9 °C) cannot be liquefied by pressure alone at room temperature but a modest degree of cooling to the ice-point at 0 °C allows the gas to be liquefied by a pressure of 40 atmospheres. There were, however, what were called informally the ‘permanent gases’, such as hydrogen, nitrogen and oxygen (and so air also), for which the necessary degree of preliminary cooling was large but still unknown. This interplay of pressure and temperature is the recurrent theme of this chapter.