Note:  Do not rely on this information. It is very old.

Critical Temperature

Critical Temperature, in Physics, is that temperature for any special gas, above which it cannot be liquefied, however great the pressure to which we subject it. Every substance may, therefore, be regarded as truly gaseous at temperatures above the critical point. Thus, the critical point for carbon dioxide is 30.9° C, a result obtained in 1869 by Dr. Andrews, to whom much of our knowledge of this subject is due. The critical points of hydrogen, oxygen, and nitrogen are all very low; hence, at ordinary temperatures they may be well called permanent gases.