HISTORY OF COLD AND THE ABSOLUTE ZEEO. 215 



exhaustion he lowered its boilino- point from minus 7(S^ C. to minus 

 110 ' C, and by (•oni))ining this low temperature with pressure all the 

 gases were liquetied b}- the year 1841, with the exception of the three 

 elementary gases — hydrogen, nitrogen, and oxygen, and three com- 

 pound gases — carbonic oxide, marsh gas, and nitric oxide. Andrews, 

 some twenty-tive years after the work of Faraday, attempted to induce 

 change of state in the uncondensed gases by using much higher pres- 

 sures than Farada}" employed. Combining the temperature of a solid 

 carbonic acid bath with pressures of 30(» atmospheres, Andrews found 

 that none of these gases exhibited any appearance of liquefaction in 

 such high states of condensation; but so far as change of volume 

 by high compression went, Andrews confirmed the earlier work of 

 Natterer ))y showing that the gases become proportionately less com- 

 pressible with growing pressure. While such investigations were 

 proceeding, Regnault and Magnus had completed their refined inves- 

 tigations on the laws of Boyle and Gay-Lussac. A very important 

 series of experiments was made by Joule and Kelvin ''On the thermal 

 efi'ects of fluids in motion " al)out 1862, in which the thermometrical 

 eflects of passing gases under compression through porous plugs fur- 

 nished important data for the study of the mutual action of the gas 

 molecules. No one, however, had attempted to make a complete study 

 of a liquefiable gas throughout wide ranges of temperature. This was 

 accomplished by Andrews in 1869, and his Bakerian lecture ''On the 

 continuity of the gaseous and liquid states of matter" will always be 

 regarded as an epoch-making investigation. During the course of this 

 research Andrews observed that liquid carbonic acid raised to a 

 temperature of 31^ C. lost the sharp concave surface of demarcation 

 between the liquid and the gas, the space being now occupied by a 

 homogeneous fluid which exhibited, when the pressure was suddenly 

 diminished or the temperature slightly lowered, a peculiar appearance 

 of moving or flickering striae, due to great local alterations of density. 

 At temperatures above 31'^ C. the separation into two distinct kinds 

 of matter could not be eflected even when the pressure reached 400 

 atmospheres. This limiting temperature of the change of state from 

 gas to liquid Andrews called the critical temperature. He showed 

 that this temperature is constant, and difi'ers with each substance, and 

 that it is alwa^'s associated with a definite pressure peculiar to each 

 bod}^ Thus the two constants, critical temperature and pressure, 

 which have been of the greatest importance in subsequent investiga- 

 tions, came to be defined, and a complete experimental proof was 

 given that "the gaseous and liquid states are only distinct stages of 

 the same condition of matter and are capable of passing into one 

 another by a process of continuous change." 



In 1873 an essay '"On the continuity of the gaseous and li([uid state," 

 full of new and suggestive ideas, was published by van der Waals, who, 

 recognizing the value of Clausius's new conception of the virial in 



