But in 1844 Faraday returned to them, armed now with new weapons, in the way of better air-pumps and colder freezing mixtures, which the labors of other workers, chiefly Thilorier, Mitchell, and Natterer, had made available. With these new means, and without the application of any principle other than the use of cold and pressure as before, Faraday now succeeded in reducing to the liquid form all the gases then known with the exception of six; while a large number of these substances were still further reduced, by the application of the extreme degrees of cold now attained, to the condition of solids. The six gases which still proved intractable, and which hence came to be spoken of as "permanent gases," were nitrous oxide, marsh gas, carbonic oxide, oxygen, nitrogen, and hydrogen. These six refractory gases now became a target for the experiments of a host of workers in all parts of the world. The resources of mechanical ingenuity of the time were exhausted in the effort to produce low temperatures on the one hand and high pressures on the other. Thus Andrews, in England, using the bath of solid carbonic acid and ether which Thilorier had discovered, and which produces a degree of cold of--80° Centigrade, applied a pressure of five hundred atmospheres, or nearly four tons to the square inch, without producing any change of state. Natterer increased this pressure to two thousand seven hundred atmospheres, or twenty-one tons to the square inch, with the same negative results. The result of Andrews' experiments in particular was the final proof of what Cagniard de la Tour had early suspected and Faraday had firmly believed, that pressure alone, regardless of temperature, is not sufficient to reduce a gas to the liquid state. In other words, the fact of a so-called "critical temperature," varying for different substances, above which a given substance is always a gas, regardless of pressure, was definitively discovered. It became clear, then, that before the resistant gases would be liquefied means of reaching extremely low temperatures must be discovered. And for this, what was needed was not so much new principles as elaborate and costly machinery for the application of a principle long familiar--the principle, namely, that an evaporating liquid reduces the temperature of its immediate surroundings, including its own substance.