206 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



all their substance. Agitation sufficient to produce even the Brown- 

 ian 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 

 arbitrary and hypothetical than the new conception. 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 good guide in such matters; 

 a theory not obviously illogical should be judged by its usefulness. 

 Let us then test the new hypothesis by applying it to other aspects 

 of physical chemistry. 



If pressure produces a change in the sizes of the atoms and mole- 

 cules themselves, may not the actual volumes of liquids and solids be 

 used as a guide to the unknown internal pressures within them? 

 Can not 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 combining several elements in succession with a single very 

 compressible one. The changes of volume occurring during the for- 

 mation of oxides were first computed; later, chlorides and bromides 

 were studied. According to the theory of compressible atoms, we 

 should expect to find greater contraction in cases of greater affinity. 

 The diagram (fig. 2), which depicts typical data concerning certain 

 nearly related chlorides, strongly supports this inference. 1 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 affinity. 

 Since chemical attraction holds two elements firmly together, why 

 should it not exert pressure ? And if it exerts pressure, why should 

 not the volume of the system be diminished by this pressure ? 



This interpretation is not wholly new. Faraday's great teacher, 

 Davy, 2 pointed out for the first time a similar fact, namely, that the 

 contraction which takes place on forming the oxide of potassium is 



i Richards, Proceedings American Academy, 1902, vol. 37, p. 399; also, especially, Richards and Jones, 

 Journal American Chemical Society, 1909, vol. 31, p. 188. 

 * Humphry Davy, Collected Works, 1840, vol. 5, p. 133 (footnote). 



