544 



SCIENCE 



[N. S. Vol. XXXIV. No. 878 



that the contraction which takes place on 

 forming the oxide of potassium is greater 

 than the contraction which takes place on 

 forming several other oxides, and he as- 

 cribed this effect to the well-known differ- 

 ences of affinity in these cases ; but he did not 

 carry the idea further. Long afterwards, 

 Braun,-^ Mueller-Erzbach,^- Hagemann^^ 

 and Traube-* independently and appar- 

 ently without knowledge of each other's 

 work, called attention to other cases of sim- 

 ilar relationships. 



All of these researches have produced so 

 little effect on the literature of the sub- 

 ject-^ that they were entirely overlooked 

 during the earlier part of the present in- 

 vestigation. The oversight mattered little, 

 however, because the whole subject needed 

 a fresh attack. Essential factors in the 

 situation had not been noticed by any of 

 these earlier investigators. AiSnities, in- 

 deed, had been considered, but the nature 

 of the substances on which the affinities act 

 had been overlooked. Evidently the 

 change of volume in any case must depend 

 not only on the intensity of the pressure 

 exerted by the affinity, but also, among 

 other things, on the compressibility of the 

 substances concerned. The greater the 

 compressibility, the greater should be the 

 change of volume caused by a given pres- 

 sure of affinity. Before any definite con- 

 clusion can be drawn, the differences in 

 compressibility must be taken into account. 



These thoughts led to the measuring of 

 the compressibilities of a large number of 



^V. Braun, see Johnson, J. Chem. Soe. Trans., 

 1877, 31, p. 252. 



= Mueller-Erzbach, Ber., 1881, 14, pp. 217, 2043. 



^ Hagemann (private publication, Friedlander, 

 Berlin, 1900). 



^ Traube, ' ' TJeber den Eaum der Atome, ' ' 

 Ahrens's Sammlung der chem. und chem.-techn. 

 Vortrage, IV., p. 256. 



'^ See, for example, Ostwald 's Grundriss der 

 allgemeinen Chemie, 1899, p. 185. 



elements and simple compounds. The pre- 

 viously employed methods for solids and 

 liquids being unsatisfactory, a new and 

 highly satisfactory method was devised for 

 the work done at Harvard. Pure mercury 

 is compressed in a suitable tube, measuring 

 both pressure and change of volume, and 

 then most of the mercury is displaced by 

 the substance to be studied, again noting 

 the relationship of pressure to volume. 

 The difference between the compressibility 

 of mercury and that of the substance is 

 then easily calculated. Obviously, in such 

 a method as this, the compressibility of the 

 apparatus itself is eliminated. The rela- 

 tion of volume to pressure is easily deter- 

 mined by causing the mercury meniscus to 

 make electrical contact with a very fine 

 platinum point in a tube of narrow diam- 

 eter, adding weighed globules of mercury, 

 and noting the corresponding pressures."" 

 Time forbids the description of the details 

 of the procedure. 



The compressibilities of thirty-five ele- 

 ments and many simple compounds were 

 studied by this method with sufficient care 

 to leave no doubt as to their relative values. 

 It became at once manifest that the forma- 

 tion of a compound of a compressible ele- 

 ment was attended with greater decrease of 

 volume than the formation of a similar 

 compound of a less compressible element, 

 other things being equal.^^ This is just 

 what the theory leads us to expect, and is a 

 fact inexplicable by any other hypothesis 

 as yet known to me. 



Another essential aspect of the theory of 

 compressible atoms is that which concerns 



" Eiehards, in collaboration with Stull, Bonnet, 

 Brink, Mathews, Jones, Speyers, Fubl. Carnegie 

 Inst, of Washington, Nos. 7 and 76; J. Amer. 

 Chem. Soc., 1904, 26, p. 399; 1909, 31, p. 154; 

 Zeitsch. physical. Chem., 1904, 49, p. 1; 1907, 61, 

 p. 77. 



'"Eiehards, Free. Amer. Acad., 1904, 39, p. 581. 



