_The Approach to the Absolute Zero._--The achievement of Kamerlingh Onnes has brought about the realization of a temperature removed only 3° from the absolute zero, and the question naturally suggests itself whether there is any probability of a still closer approach to that point. The answer is that if, as is not impossible, there exists a gas, as yet unisolated, which has an atomic weight one-half that of helium, that gas, liquefied in turn by the aid of liquid helium, would render that approach possible, though the experimental difficulties of the operation would be enormous and perhaps prohibitive. The results of experiments bearing on this question and of theory based on them are shown in table II. The third column shows the critical temperature of the gas which can be liquefied by continuous expansion through a regenerative cooling apparatus, the operation being started from the initial temperature shown in the second column, while the fourth column gives the temperature of the resulting liquid. It will be seen that by the use of liquid or solid hydrogen as a cooling agent, it should be possible to liquefy a body having a critical temperature of about 6° to 8° on the absolute scale, and a boiling point of about 4° or 5°, while with the aid of liquid helium at an initial temperature of 5° we could liquefy a body having a critical temperature of 2° and a boiling point of 1°. TABLE II. +------------------+-------------+-------------+---------------+ | | Initial | Critical |Boiling Points.| | Substance. | Temperature.|Temperature. | Abs. Degrees. | | |Abs. Degrees.|Abs. Degrees.| | +------------------+-------------+-------------+---------------+ | (Low red heat) | 760 | 304 | 195 (CO2) | | (52° C.) | 325 | 130 | 86 (Air) | | Liquid air under | | | | | exhaustion | 75 | 30 | 20 (H) | | Liquid hydrogen | 20 | 8 | 5 (He) | | Solid hydrogen | 15 | 6 | 4 | | Liquid helium | 5 | 2 | 1 | +------------------+-------------+-------------+---------------+