FUNDAMENTAL. PEOPEETIES OF THE ELEMENTS RICHARDS. 209 



pressed and possess small volume, whereas molecules with a slight 

 cohesive affinity should be more bulky. Moreover, those molecules 

 already much compressed by their own self-affinity would naturally 

 be but little affected by additional pressure. Thus, as regards two 

 substances otherwise similar, the less volatile one would be less com- 

 pressible, denser, and possess greater surface tension. 1 These out- 

 comes of the theory agree with the facts in 80 per cent of the cases thus 

 far studied; for example, o-xylene is denser, less volatile, less com- 

 pressible, and possesses a greater surface tension than either m-xylene 

 or p-xylene. 2 Differences of structure and differences of chemical 

 nature sometimes conceal these relations; the parallelism appears 

 most strikingly among isomeric compounds. In brief, the bulk of 

 evidence strongly indicates that cohesiveness as well as chemical 

 affinity exerts pressure in its action, and hence that each plays a part 

 in determining the volumes occupied by molecules. 



Thus the computation of the space occupied by either a solid or a 

 liquid becomes a very complex matter. Not only must the various 

 chemical affinities at work be taken into account, but also the co- 

 hesive attraction of both factors and products, and the compressi- 

 bilities over a very wide range of all the substances concerned. Dis- 

 coverable parallelism in volume changes is to be expected only when 

 one alone of these tendencies is the chief variable. 



The exact mathematical working out of the consequences is very 

 far in the distance, if, indeed, it can ever be attained. This fact does 

 not, however, militate in the least against the plausibility of the idea. 

 Although mankind has not yet been able to devise a method of mathe- 

 matical analysis which will solve at one stroke the gravitational rela- 

 tions of three bodies, nature is not on that account prevented from 

 causing three or more bodies to act on one another with the force of 

 gravity, or astronomers from calculating as nearly as may be the 

 consequences by a process of approximation. 



Carried through to its logical conclusion, the idea that atoms are 

 compressible gives one quite a new conception of the molecular me- 



1 Richards and Mathews, Zeitsch. physikal. Chem., 190S, vol. 61, p. 449. 



2 With the help of C. L.SpeyersI have determined these constants with great care. The substances were 

 unusually pure, the p-xylene freezing at 13.2°. The details will be published as soon as possible. There- 

 suits are recorded in the following table: 



Boiling 

 point. 



Density 



2074" 



Surface 



tension 



mg./mm.; 20' 



Compres- 

 sibility X 

 10 6 at 20° 



per 

 megabar. 



o-xylene 



m-xylene 



p-xylene 



38734°— sm 1911 14 



144.0° 



139.0 



136.2 



0. 8811 

 .8658 

 .8611 



3.09 

 2.96 

 2.92 



61.1 

 64.8 



C6.8 



