Explanatory Notes 113 



between them, so that never would the effect of pressure 

 be assisted by the action of internal forces. In all actual 

 gases the particles have some intrinsic size of their own; 

 and cohesion becomes sooner or later conspicuous, so that 

 ultimately liquefaction is possible. The further a gas is 

 from showing any sign of molecular cohesion, the more 

 ' nearly is it considered "perfect" as a gas. An imperfect 

 gas exhibits already an incipient tendency towards ulti- 

 mate liquefaction. A corrected form of Boyle's law, applic- 

 able not only to imperfect gases but also to liquids, was 

 devised by the great Dutch physicist Van der Waals, whose 

 treatment, though not even yet finally accurate, was pro- 

 foundly interesting and instructive. 



Page 19 



The way in which Kepler discovered his famous laws, 

 from elaborate discussions of the planetary observations of 

 Tycho Brahe, is explained in many books: among others, 

 in the author's own semi-popular volume called Pioneers 

 of Science (Macmillan). 



Page 21 



The familiar behaviour of a wet sandy beach, when 

 walked over, is very curious; the pressure of a foot dries it, 

 while relaxation of pressure moistens it. In other words, 

 pressure applied to a collection of granular particles tends 

 to increase the spaces between them, and so enables them 

 to hold more water in their interstices. Relaxation and 

 constraint allow the molecules to adjust themselves closer 

 together, and so to squeeze some of the liquid out again. 

 Just opposite to the behaviour of a sponge. 



It was on a highly developed superstructure based on 

 this foundation that Professor Osborne Reynolds of Man- 

 chester devised his scheme, wherein "the ether" was sup- 



8 



