Other workers took up the subject, notably Wroblewski and Olszewki in Poland, Dewar in England, and Kammerlingh Onnes in Holland; and the liquefaction of all the gases has now been accomplished. The following table shows the _absolute_ boiling-(B.P.) and melting-points (M.P.), critical points (C.P.), and pressures (C.Press.), together with the density (D) at their boiling-points of a number of liquefied gases: C. B. P. M. P. C. P. Press. D. _degrees_ _degrees_ _degrees_ _m._ Helium 4.5 —— —— —— 0.15 Hydrogen 20 15 35 11.6 0.06 Oxygen 90.5 below 50 154 44 1.131 Nitrogen 77.5 60 124 20.9 0.791 Methane 108.3 —— 191 42.4 0.416 Ethylene 169.5 104 282 44 0.571 Fluorine 186 40 —— —— 1.11 Chlorine 239.6 —— —— —— 1.507 Ammonia 234.5 197.5 404 85.9 —— Neon 30.40 —— below 65 —— —— Argon 86.90 —— 155.6 40.2 1.212 Krypton 121.33 —— 210.5 41.2 2.155 Xenon 163.9 —— 287.8 43.5 3.52 The principle of the Cailletet method of effecting the liquefaction of oxygen had been theoretically and experimentally studied by Joule and Lord Kelvin many years previously. It was extended by Siemens and has been applied by Linde and Hampson to the construction of machinery for the production of liquid air on a large scale, without the use of any intermediate refrigerant. It is now readily possible to procure considerable quantities of liquid air, and even of liquid hydrogen. By the evaporation of liquid hydrogen temperatures approaching the _absolute zero_—that is, 273° C. below the melting-point of ice—can now be reached. Incidentally there has been developed a special field of inquiry relating to the behaviour of substances at low temperatures. [Illustration: SIR JAMES DEWAR.] The pioneers in this field have been Dewar in England and Kammerlingh Onnes in Holland. Research at low temperatures, indeed, has been the main feature of the work of the Royal Institution of Great Britain during the last twenty years. It has included observations at temperatures approaching the absolute zero, on the electrical resistivity of metals and alloys, on the behaviour of so-called insulators, on changes in the cohesive force of metals, on the dielectric constants of frozen electrolytes, on the influence of cold on magnetisation and on magnetic permeability, and on the optical behaviour of bodies, on vital phenomena at low temperatures, and on the influence of cold on chemical change.