October 27, 1911] 



SCIENCE 



545 



cohesion.-^ If the pressure of chemical 

 affinity causes atomic compression, may not 

 the pressure of cohesive affinity also have 

 the same effect? Traube suggested this 

 possibility, but looked at the whole question 

 from a different point of view.-" The affin- 

 ity which prevents solids and liquids from 

 vaporizing is generally admitted to produce 

 great internal pressure ; must it not tend to 

 compress the molecules into smaller space? 

 Molecules with high cohesive affinity (those 

 of substances hard to volatilize) should be 

 much compressed 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 

 compressible, denser and possess greater 

 surface tension.^" These outcomes of the 

 theory agree with the facts in eighty per 

 cent, of the cases thus far studied; for ex- 

 ample, o-xylene is denser, less volatile, 

 less compressible and possesses a greater 

 surface tension than either m-xylene or 

 p-xylene.^^ Differences of structure and 



^ Ibid. 



^° See especially Traube, Ann. Physih., 1897, 

 (III.), 61, p. 383; 1901, (IV.), 5, p. 548; 1902, 8, 

 p. 267; 1907, 22, p. 519; Zeitsch. physilcal. Chem., 

 1910, 68, p. 289; also Walden, Zeitsch. physilcal. 

 Chem., 1909, 66, p. 385. Their interpretation de- 

 pends largely on the application of van der 

 Waals's equation and the complicating assumption 

 of a co-volume; however, Walden 'a very recent 

 paper presents a number of interesting and impor- 

 tant relations concerning internal pressure, which 

 seem to demand the assumption of atomic com- 

 pressibility for their explanation. 



°° Eichards and Mathews, Zeitsch. physilcal. 

 Chem., 1908, 61, p. 449. 



""^With the help of C. L. Speyers I have deter- 

 mined these constants with great care. The sub- 

 stances were unusually pure, the p-xylene freezing 

 at 13.2°. The details will be published as soon as 



differences of chemical nature sometimes 

 conceal these relations; the parallelism ap- 

 pears most strikingly among isomeric com- 

 pounds. 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 mole- 

 cules. 



Thus the computation of the space occu- 

 pied 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 cohesive attrac- 

 tion of both factors and products, and the 

 compressibilities over a very wide range of 

 all the substances concerned. Discoverable 

 parallelism in volume changes is to be ex- 

 pected only when one alone of these forces 

 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 mathematical analysis 

 which will solve at one stroke the gravita- 

 tional relations 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 approxima- 

 tion. 



Carried through to its logical conclusion, 

 the idea that atoms are compressible gives 

 one quite a new conception of the molecular 



possible. The results are recorded in the follow- 

 ing table: 



Surface Compres- 

 Boiling Density, Tension sibility 

 Point 20°/4° mg./mm.;20° 10«at20° 



0-Xylene . 144.0° 0.8811 3.09 60.0 



ni-Xylene . 139.0 0.8658 2.96 63.5 



p-Xylene . 136.2 0.8611 2.92 66.2 



