734 Professor Deivar [Jan. 18, 



in his 'Mathematical Physics,' published in 1847, made a consider- 

 able advance in the application of the theory. Joule made a great 

 step in 1821 by calculating the mean translational velocity of the 

 particles of hydrogen required to produce in a closed space the pres- 

 sure of one atmosphere at the melting point of ice; but the great 

 advance in the application of the theory was due to Clausius, ably sup- 

 ported later on by Maxwell, Boltzmann, Meyer and Van der Waals. 



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 gases through porous 

 pings furnished important data for the study of the mutual action of 

 the gas molecules. 



Such experiments, along with a knowledge of the specific and 

 latent heat, together with the rate of diffusion, viscosity and thermal 

 conductivity, furnished material for a complete thermo-dynamical 

 treatment of the gaseous state. Professor Van der Waals entered 

 upon this difficult inquiry in 1873 by publishing an essay ' On the 

 Continuity of the Gaseous and Liquid States,' full of new and sug- 

 gestive ideas. 



The equation of continuity Van der Waals developed 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 for 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 pres- 

 sure and the critical volume. In the case of carbonic acid the theo- 

 retical 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 impor- 

 tant paper on the isothermals of oxygen, nitrogen, hydrogen, ethy- 

 lene, etc., in the year 1880. It now became possible to calculate 

 the critical data with comparative accuracy for the gases oxygen and 

 nitrogen. This was done by Sarrau in 1882, and the subsequent 

 static liquefaction of oxygen by Wroblewski in 1883 confirmed the 

 theoretical conclusions. No doubt a great impulse had been given 

 to research in this department by the suggestive experiments of 

 Pictet and Cailletet in 1878. 



The theory of Van der Waals has been of the greatest importance 

 in directing experimental investigation in attacking the difficult 

 problem of the liquefaction of the permanent gases. In the space 

 of an hour's lecture it is impossible to do justice to all the workers 

 who have contributed materially to the advance of this department. 



