648 



SCIENCE PROGRESS 



Some Points connected with the Liquid State (Gervaise Le Bas, B.Sc.) 



Some have supposed that there is a sort of compactness in the arrangements 

 of the molecules in the liquid state. Whether this be so or not, there is a change 

 from the conditions obtaining in the vapour state. It is well known that pv 

 where p is the vapour pressure and v the molecular volume, is, according to 

 the kinetic theory, a measure of the free space between the molecules. Two 

 conditions alter the perfect gas relationships : 



(a) The molecules have a definite size. 



(b) There are mutual attractions. 



This causes the values of pv for each substance, as they are traced to absolute 

 zero, to be along curves similar to those found in the diagrams. The second 



400 25 50 75 500 



AbsoMe Temperature . 



condition exists up to the critical point. The result is that the values oi pv, and 

 thus the free space, diminishes to absolute zero, when the points corresponding to 

 each substance lie along a" b" . 



It is found that at the points similar to b", the curves cut a" b" almost, if not 

 quite, at right angles. It follows that, owing to the steepness of the curves at b", 

 the rate of diminution of the free space with temperature is enormous, if not 

 infinite. The free space is then not zero, but a definite fraction of the space. Some 

 circumstance thus prevents the molecules from collapsing to their volumes at 

 absolute zero. It is not the vibration of the molecules as such, for, as already 

 stated, the rate of variation of this with temperature is very great. The reason 

 is probablyto be sought in the atomic vibrations, which cause the molecules to fill 

 up the space available. The volume in the liquid gradually diminishes to that at 

 absolute zero owing to the gradual diminution of the atomic spheres of activity 



