RICHARDS. — SIGNIFICANCE OF CHANGING ATOMIC VOLUME. 9 



oxygen the atomic critical volume, the contractions are as follows : 

 14.7 + 24.5 — 19.4 = 19.8, in the case of mercury, and 9.5 + 24.5 — 

 14.5 = 19.5, in the case of zinc. If the metals were originally subject 

 to the same internal stress, we should infer from the similarity of con- 

 tractions that the affinities concerned in the two cases were about equal. 

 This inference is, however, overthrown by other facts. Both elements 

 have about the same atomic heat capacity, hence no internal rearrange- 

 ment takes place in one which is not approximated in the other. On 

 the other hand, the increase in atomic volume for a rise of 1° of tem- 

 perature exhibited by one is much greater than that exhibited by the 

 other. 



If a gram atom of one element increases more rapidly in size than 

 the gram atom of another, it is only reasonable to suppose that the 

 heat energy is finding less opposition in the former case. The co- 

 efficient of cubic expansion of mercury is 0.000179 at 0°C. and the 

 heat required to raise a gram through 1° is 0.139 joule. With zinc 

 the corresponding numbers are 0.000087 and 0.392. * The respective 

 atomic volumes are 14.7 and 9.5. Substituting these values in the 

 equation we obtain. 



„ (200X0.139) T^.^nn A 



""' "^ (14.7 X 0.000179) "^ 1^^'^^^ megadynes per square cm. 



^^" ^ (9.5 X 0.000087) ^ ^l^'^O^ megadynes per square cm. 



Both these pressures are very large, for a megadyne exerts on a 

 square centimeter a pressure of almost an atmosphere. As has been 

 said, they signify a resultant tendency which resists expansion. 



It is interesting to note that these stresses agree in their indications 

 with the comparison of boiling points and latent heats of evaporation. 

 The boiling point of mercury is 357° C. and that of zinc about 930° C. 

 The latent heat of evaporation of zinc is not known, but there is no 

 reason for believing that in its case Trouton's rule is broken. Hence 

 the criteria all indicate that zinc is harder to dissociate from itself than 

 mercury is. 



A comparison of the energy-quotients of several metals, measured in 

 this way, may be of interest. 



* All figures not otherwise designated were taken from the tables of Landolt 

 and BiJrnstein, 1894. 



