One of the most skillful and successful experimenters in heat was also a Frenchman, Henri Victor Regnault (1810–78). He greatly improved the construction and use of the thermometer, and was the first to discover that the indications of an air thermometer and one of mercury did not exactly agree, because they did not expand in the same degree for equal increases of temperature. His most important work was on the expansion of gases, vapor pressure, specific heat of water, etc., and for careful, patient measuring he had a positive genius. Until he proved the contrary it had been assumed that all gases had the same coefficient of expansion, and Boyle’s law that the volume of a gas was inversely proportional to its pressure had not been questioned. His tables of the elastic force of steam have been of immense practical value, but his studies of the expansion of gases are of greater interest because they have pointed the way to one of the most important accomplishments of the century, the liquefaction of all known gases. During the earlier years of this century it was the custom to consider vapors and gases as quite distinct forms of matter. Vapors always came, by evaporation, from liquids, and could always be “condensed” or reduced to the liquid form without difficulty, but it was not thought possible to liquefy the so-called “permanent” gases. The first man to attack the problem systematically was Michael Faraday, who, before the end of the first third of the century, had liquefied several gases, mostly by producing them by chemical reactions under pressure. Several of the more easily reducible gases or vapors, such as ammonia, sulphurous acid, and probably chlorine, had been previously liquefied by cold, but a quarter of a century elapsed after Faraday’s researches before the true relation of the liquid and gaseous states of matter was understood, and it was found that both increase of pressure and lowering of temperature were, in general, essential to the liquefaction of a gas. It was Thomas Andrews, of Belfast, who first showed, in a paper published in 1863, that there was a continuity in the liquid and gaseous states of matter, that for each substance there was a critical temperature at which it became a homogeneous fluid, neither a liquid nor a gas: that above this temperature great pressure would not liquefy, while below it the substance might exist as partly liquid and partly gas. He pointed out the fact that for the so-called permanent gases this critical temperature must be exceedingly low, and if such temperature could be reached liquefaction would follow.