Vapour Pressure and Affinity of Isotopes. 17& 



much larger number of nerves and consequent conflict with 

 some anatomical results are thus removed. 



5. Any hypothesis like the resonance theory in question 

 must be very difficult to prove to the hilt by any number of 

 confirmatory experiments. But, it' it is essentially at variance 

 with facts, its disproof should be comparatively easy. 



Nottingham, 

 March 20, 1919. 



XII. Note on the Vapour Pressure and Affinity of Isotopes. 

 By F. A. LlNDEMANN, Ph.I)* 



IT was shown in a recent paper that isotopes must be 

 separable both by fractionation and by chemical means t.. 

 At that time it was not possible to give any quantitative 

 estimate of the differences in vapour pressure or affinity^ 

 since data were lacking as to the physical properties. This 

 omission may now be repaired, since the melting-points of 

 two sorts of lead of atomic weight 207*19 and 206*34 have 

 been found to be identical within the limits of error of the 

 experiment J. 



It lias been shown that the melting-point may be regarded 

 as the temperature at which the average amplitude of oscil- 

 lation of the atoms is equal to the distance between them §. 

 This distance is obviously the distance between the centres 

 of two neighbouring atoms minus the diameter of the atom 

 which is presumably defined by some electronic orbit. Since 

 the spectra of isotopes are practically identical, the outer elec- 

 tronic orbits are presumably identical, so that the diameters 

 of the atoms may be regarded as identical. On the other 

 hand, the densities are proportional to the atomic weights ||, 

 from which we may conclude that the distances between the 

 centres of neighbouring atoms are identical. It follows 

 from this that the amplitudes of oscillation at the melting- 

 point are identical. 



As is well-known there are two alternative views which 

 give the same formula for the atomic heat. According to 

 the first, a linear oscillator of frequency v can only absorb or 



* Communicated by the Author. 



t F. A. Lindemann & F. W. Aston, Phil. Mag-, xxx\ ii. p. 523 i 1910). 

 J T. W, Richards, ' Presidential Address to American Association for 

 the Advancement of Science ' (1918). 



§ F. A. Lindemann, Phys. Zeitschrift, xi. p. 009 (1910). 

 || T. W. Richards, he. eit. 



