ADDRESS. 29 



van der Waals equation has been made by a lady, Mme. Christine 

 Meyer. The idea at the base of this new development may be under- 

 stood from the following general statement : van der "Waals brings 

 the van der Waals surfaces for all substances into coincidence at the 

 point where volume, pressure, and temperature are nothing, and then 

 stretches or compresses all the surfaces parallel to the three axes of 

 volume, pressure, and temperature, until their critical points coincide. 

 But on this plan the surfaces do not quite coincide, because the points 

 where the three variables are respectively nothing are not corresponding 

 points. Mme. Meyer's plan is to bring all the critical points first 

 into coincidence, and then to compress or extend all the representative 

 surfaces parallel to the three axes of volume, pressure, and temperature, 

 until the surfaces coincide. In this way, taking twenty-nine different 

 substances, she completely verifies from experiment van der Waals' law 

 of correspondence. The theory of van der Waals has been one of the 

 greatest importance in directing experimental investigation, and in 

 attacking the difficult problems of the liquefaction of the most permanent 

 gases. One of its greatest triumphs has been the proof that the^ critical 

 constants and the boiling-point of hydrogen theoretically deduced by 

 Wroblewski from a study of the isothermals of the gas taken far above 

 the temperature of liquefaction are remarkably near the experimental 

 values. We may safely infer, therefore, that if hereafter a gas be 

 discovered in small quantity even four times more volatile than liquid 

 hydrogen, yet by a study of its isothermals at low temperature we shall 

 succeed in finding its most important liquid constants, although the 

 isolation of the real liquid may for the time be impossible. It is perhaps 

 not too much to say that as a prolific source of knowledge in the depart- 

 ment dealing with the continuity of state in matter, it -w^ould be necessary 

 to go back to Carnot's cycle to find a proposition of greater importance 

 than the theory of van der Waals and his development of the law of 

 corresponding states. 



It will be apparent from what has just been said that, thanks to the 

 labours of Andrews, van der Waals, and others, theory had again far 

 outrun experiment. We could calculate the constants and predict some 

 of the simple physical characteristics of liquid oxygen, hydrogen, or 

 nitrogen with a high degree of confidence long before any one of the 

 three had been obtained in the static liquid condition permitting of the 

 experimental verification of the theory. This was the more tantalising, 

 because, with whatever confidence the chemist may anticipate the sub- 

 stantial corroboration of his theory, he also anticipates with almost 

 equal conviction that as he approaches more and more nearly to the 

 zero of absolute temperature, he will encounter phenomena compelling 

 modification, revision, and refinement of formulas which fairly covered 

 the facts previously known. Just as nearly seventy years ago chemists 

 were waiting for some means of getting a temperature of 100 degrees 

 below melting ice, so ten years ago they were casting about for 



