OCTOBEK 27, 1911] 



SCIENCE 



543 



throughout all their substance. Agitation 

 sufficient to produce even the Brownian 

 movement might easily exist in such a 

 system. 



Clearly there is nothing impossible or 

 obviously contradictory to experimental 

 knowledge in the notion that atoms are 

 compressible ; indeed, the old idea of small, 

 hard particles far apart is really more arbi- 

 trary and hypothetical than the new con- 

 ception. The obvious simplicity of the 

 latter is rather in its favor than otherwise, 

 as in Dalton's atomic theory. In general, 

 the more simply an hypothesis interprets 

 the phenomena of nature, the more useful 

 the hypothesis is likely to be, provided, of 

 course, that the interpretation is adequate. 

 The modern philosophy of pragmatism is a 



COMPARISON of HEATS OF FORMATION of ( HLORIDES 

 and CONTRACTION ON (OMBINATION 



CONTRACTION 

 ON COMBINATION 



good guide in such matters; a theory not 

 obviously illogical should be judged by its 

 usefulness. Let us then test the new hy- 

 pothesis by applying it to other aspects of 

 physical chemistry. 



If pressure produces a change in the 

 sizes of the atoms and molecules themselves, 

 may not the actual volumes of liquids and 

 solids be used as a guide to the unknown 



internal pressures within them? Cannot 

 we thus discover whether or not chemical 

 affinity exerts pressure in its action? To 

 follow this clue, the simplest possible case 

 was chosen at first, namely, the comparison 

 of the contractions taking place on com- 

 bining several elements in succession with a 

 single very compressible one. The changes 

 of volume occurring during the formation, 

 of oxides were first computed; later, chlo- 

 rides and bromides were studied. Accord- 

 ing to the theory of compressible atoms, we- 

 should expect to find greater contraction in 

 eases of greater affinity. The diagram 

 (Pig. 2), which depicts typical data con- 

 cerning certain nearly related chlorides, 

 strongly supports this inference.^" One of 

 these lines shows the total change of volume 

 which occurs when a gram-molecule of 

 chlorine combines with the equivalent 

 weight of metal; the other gives the heat 

 evolved during combination. The lines 

 show distinct parallelism; that is to say, 

 reactions evolving much heat manifest 

 great contraction. In cases of this kind 

 the heat of reaction is usually not very 

 different from the change of free-energy, 

 therefore we may infer that greater affinity 

 is associated with greater contraction; and 

 it is but a small leap in the dark to guess 

 that the change of volume is caused by the 

 pressure of afSnity. Since chemical attrac- 

 tion holds two elements firmly together, 

 why should it not exert pressure ? And if 

 it exerts pressure, why should not the vol- 

 ume of the system be diminished by this 

 pressure ? 



This interpretation is not wholly new. 

 Faraday's great teacher, Davy,^" pointed 

 out for the first time a similar fact ; namely, 



"Richards, Proc. Amer. Acad., 1902, 37, p. 399 j 

 also especially J. Amer. Chem. Soc, 1909, 31, p. 

 188. 



=» Humphry Davy, "Collected "Worte," 1840, 

 5, p. 133 (foot-note). 



