For the technical scientists themselves, however, the low-temperature field is still full of inviting possibilities of a strictly technical kind. The last gas has indeed been liquefied, but that by no means implies the last stage of discovery. With the successive conquest of this gas and of that, lower and lower levels of temperature have been reached, but the final goal still lies well beyond. This is the north pole of the physicist's world, the absolute zero of temperature--the point at which the heat-vibrations of matter are supposed to be absolutely stilled. Theoretically this point lies 2720 below the Centigrade zero. With the liquefaction of hydrogen, a temperature of about -253 deg or -254 deg Centigrade has been reached. So the gap seems not so very great. But like the gap that separated Nansen from the geographical pole, it is a very hard road to travel. How to compass it will be the study of all the low-temperature explorers in the immediate future. Who will first reach it, and when, and how, are questions for the future to decide. And when the goal is reached, what will be revealed? That is a question as full of fascination for the physicist as the north-pole mystery has ever been for the generality of mankind. In the one case as in the other, any attempt to answer it to-day must partake largely of the nature of a guess, yet certain forecasts may be made with reasonable probability. Thus it can hardly be doubted that at the absolute zero all matter will have the form which we term solid; and, moreover, a degree of solidity, of tenacity and compactness greater than ever otherwise attained. All chemical activity will presumably have ceased, and any existing compound will retain unaltered its chemical composition so long as absolute zero pertains; though in many, if not in all cases, the tangible properties of the substance--its color, for example, and perhaps its crystalline texture--will be so altered as to be no longer recognizable by ordinary standards, any more than one would ordinarily recognize a mass of snowlike crystals as air.