1901.] on Gases at the Beginning and End of the Century. 733 



body. No wonder that Thilorier should say that inside a Faraday 

 tube was a new world in which totally unexpected phenomena 

 occurred. 



Faraday, by evaporating the carbonic acid snow in an exhausted 

 receiver, succeeded in lowering the boiling point from —78° C. to 

 — 110° C. Combining the action of this low temperature with pres- 

 sure, all gases by the year 1844 were liquefied, with the exception 

 of the three elementary gases, hydrogen, nitrogen, oxygen, and the 

 compound gases, carbonic oxide, marsh gas and nitric oxide. An- 

 drews, some twenty-five years after the work of Faraday, attempted 

 to induce change of state in the uncondensed gases, by using much 

 higher pressures than Faraday employed. Combining the tempera- 

 ture of the solid carbonic acid bath with pressures of 300 atmo- 

 spheres, Andrews found that none of the gases exhibited any 

 appearance of liquefaction even in such high states of condensation ; 

 so far as change of volume by high compression went, Andrews 

 confirmed the earlier work of Natterer, showing that the gases 

 become less and less compressible with growing pressure. While 

 such investigations were proceeding, Eegnault and Magnus had 

 made refined observations on the laws of Boyle and Gay Lussac, 

 but none had made a complete study of the isothermals of a liquefiable 

 gas through 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 strias, due to 

 great local alterations of density. 



At temperatures above 31° C. the separation into two distinct 

 kinds of matter could not be effected 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 differs with each substance, and that it 

 is always associated with a definite pressure peculiar to each body. 

 Thus the two constants, the critical temperature and pressure, 

 which have been of the greatest importance in subsequent investiga- 

 tion, 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." 



The fundamental idea that gaseous pressure was the result of a 

 succession of strokes of bombarding particles was first put forward 

 by Bernouilli about the middle of the eighteenth century. Later the 

 same suggestion was employed by Lessage, of Geneva ; and Herepath, 



