246 THE SCIENTIFIC MONTHLY
same manner. But some gases resisted liquefaction by this means, con- spicuously those existing in the atmosphere. These were called perma- nent gases. But in 1877, Pictet and Gailletet independently succeeded in obtaining a few drops of liquid air. In 1895, processes were invented by which air could be liquefied by the gallon. Hydrogen resisted lique- faction, till Dewar, in 1898, succeeded in reducing it also to the liquid form. There was left one gas only, namely helium, unliquefied. A suffi- ciently low temperature was not available. But in 1908 Omnes gained the distinction of liquefjring this last remaining gas and obtained a liquid that boils at 4.5 Centigrade degrees above the absolute zero of temperature, but that, so far, has not been solidified, though a tempera- ture as low as 2.5** absolute has been reached.
A word of explanation should perhaps be given as to what is meant by absolute temperature. The difference between our coldest winter weather and hottest summer is a little more than half the difference be- tween freezing and boiling water, which on the Centigrade thermometer is one hundred degrees. The melting point of ice is zero on the Centi- grade scale, the boiling point of water is 100° C. Bright red heat is about 1,000® ; furnaces for iron is 1,300-1,700® ; the melting point of tungsten is 3,000® ; in the electric furnace a temperature of 3,500® C. has been reached, perhaps even a higher temperature. Probably no tem- perature higher than 4,000® C. has been made by man, but the tempera- ture of the sun has been estimated at 6,000® C. There is nothing to pre- vent us conceiving of a temperature of 10,000® or even of 100,000® in the same sense as we can conceive one hundred million dollars. While, however, we have no difficulty in attaining a temperature of seven or eight hundred degrees and can attain a much higher temperature, we not only have not reached a temperature of three hundred degrees below zero, but we are almost certain that such a temperature is impossible. Several lines of argument lead to the conclusion that at — 273® C. a body would be absolutely without heat and that any lower temperature is therefore impossible. So — 273® C. is called the absolute zero and helium has been cooled to — 270.5® C. or 2.5® absolute without having been frozen. The difference in temperature of a room very slightly chilly and almost comfortably warm is about 2.5®.
Liquid helium is about one seventh as dense as water, which is ap- proximately the same ratio as the gas bears to air at the ordinary tem- perature. When boiling the volume of gaseous helium is only eleven times that of the liquid, while steam is nearly seventeen hundred times the volume of the water from which it is produced.
In 1898, Bamsay and Travers published accounts of three other gases found in the air, krypton and xenon being heavier than argon and neon being lighter. The two heavier ones were got from liquid air. Liquid air is produced on the commercial scale; one of its chief uses is
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