ADDRESS. 



27 



Willard Gibbs. The suggestive work of James Thomson undoubtedly 

 was a valuable aid to van der Waals, for as soon as he reached the point 

 where his equation had to show the continuity of the two states this was 

 the first difficulty he had to encounter, and he succeeded in giving the 

 explanation. He also gave a satisfactory reason for the existence of a 

 minimum value of the product of volume and pressure in the Regnault 

 isothermals. His isothermals, with James Thomson's completion of them, 

 were now shown to be the results of the laws of dynamics. Van der Waals 

 applied the new equation to the consideration of the coefficients of expan- 

 sion with temperature and of pressure with temperature, showing that 

 although they were nearly equal, nevertheless they were almost independent 

 quantities. His investigation of the capillarity constant was masterly, and 

 he added further to our knowledge of the magnitudes of the molecules of 

 gases and of their mean free paths. Following up the experiments of Joule 

 and Kelvin, he showed how their cooling coefficients could be deduced, 

 and proved that they vanished at a temperature in each case which is a 

 constant multiple of the specific critical temperature. The equation of 

 continuity developed by van der "Waals involved the use of three constants 

 instead of one, as in the old law of Boyle and Charles, the latter being 

 only utilised to express the relation of temperature, pressure, and volume, 

 when the gas is far removed from its point of liquefaction. Of the two 

 new constants one represents the molecular pressure arising from the 

 attraction between the molecules, the other four times the volume of the 

 molecules. Given these constants of a gas, van der Waals showed that 

 his equation not only fitted into the general characters of the isothermals, 

 but also gave the values of the critical temperature, the critical pressure, 

 and the critical volume. In the case of carbonic acid the theoretical 

 results were found to be in remarkable agreement with the experimental 

 values of Andrews. This gave chemists the means of ascertaining the 

 critical constants, provided sufficiently accurate data derived from the 

 study of a few properly distributed isothermals of the gaseous substance 

 were available. Such important data came into the possession of chemists 

 when Amagat published his valuable paper on 'The Isothermals of 

 Hydrogen, Nitrogen, Oxygen, Ethylene, &c.,' in the year 1880. It now 

 became possible to calculate the critical data with comparative accuracy 

 for the so-called permanent gases oxygen and nitrogen, and this 

 was done by Sarrau in 1882. In the meantime a great impulse had 

 been given to a further attack upon the so-called permanent gases 

 by the suggestive experiments made by Pictet and Cailletet. The 

 static liquefaction of oxygen was effected by Wroblewski in 1883, and 

 thereby the theoretical conclusions derived from van der Waals' equation 

 were substantially confirmed. The liquefaction of oxygen and air was 

 achieved through the use of liquid ethylene as a cooling agent, which 

 enabled a temperature of minus 140 degrees to be maintained by its steady 

 evaporation in vacuo. From this time liquid oxygen and air came to be 

 regarded as the potential cooling agents for future research, commanding 



