26 REPORT— 1902. 



proceeding, Regnault and Magnus had completed their refined investiga- 

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

 experiments was made by Joule and Kelvin ' On the Thermal Effects of 

 Fluids in Motion' about 1862, in which the thermometrical effects of 

 passing ga.ses under compression through porous plugs furnished 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 strife, 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 tempera- 

 ture. 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, critical temperature and pressure, 

 which have been of the greatest importance in subsequent investigations, 

 came to be defined, and a complete experimental proof was given that 

 * the gaseous and liquid states are only distinct stages of the same condi- 

 tion 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 Liquid State,*^ 

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

 recognising the value of Clausius' new conception of the Virial in Dynamics, 

 for a long-continued series of motions, either oscillatory or changing 

 exceedingly slowly with time, applied it to the consideration of the mole- 

 cular movements of the particles of the gaseous substance, and after much 

 refined investigation, and the fullest experimental calculation available at 

 the time, devised his well-known Equation of Continuity. Its paramount 

 merit is that it is based entirely on a mechanical foundation, and is in 

 uo sense empiric ; we may therefore look upon it as having a secure 

 foundation in fact, but as being capable of extension and improvement. 

 James Thomson, realising that the straight-line breach of continuous cur- 

 vature in the Andrews isothermals was untenable to the physical mind, pro- 

 pounded his emendation of the Andrews curves — namely, that they were 

 continuous and of S form. We also owe to James Thomson the concep- 

 tion and execution of a three-dimensional model of Andrews' results, which 

 has been of the greatest service in exhibiting the three variables by means 

 of a specific surface afterwards greatly extended and developed by Professor 



