FUNDAMENTAL PROPERTIES 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.t 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.? 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., 1908, vol. 61, p. 449. 
2 With the help of C. L.Speyers I have determined these constants with great care. Thesubstances were 
unusually pure, the p-xylene freezing at 13.2°. The details will be published as soon as possible. The re- 
sults are recorded in the following table: 
Compres- 
point. 20°/4 mg./mm.; 20°] per 
megabar. 
G=XYICHO NSE CER REL tee te SEE ON Cee eRe 144. 0° 0. 8811 3. 09 61.1 
STP ICTIG ie trae Sa Hts lee a di. Se ar fk bi a Shogie 139.0 . 8658 2. 96 64.8 
P-XMICHD ey Psa se ae ta EE eh Cie Lh ae See 136. 2 . 8611 2. 92 66.8 
38734°—sm 1911——14 
