296 BECKER AND DAY 



appear to lead to some interesting reflections. They certainly 

 offer a confirmation from a new standpoint of Thomson's theory 

 of solids for which so much other and more exact evidence is 

 accumulating, and in so far as the foam is comparable with a 

 true solid, it suggests some new ideas upon the nature of the 

 molecule itself. In the foam, when statical conditions are 

 reached, the molecules (bubbles) themselves are not in motion. 

 From this point of view, the molecule is merely the space 

 enclosed between a fixed set of equipotential surfaces, and what 

 has been regarded as molecular motion is confined to the cor- 

 puscles constituting the molecule, instead of being an attribute 

 of the centroid of the molecule itself. 



In the foam much is known regarding the form of these equi- 

 potential bounding surfaces. Lord Kelvin has shown that the 

 figure of stable equilibrium corresponds very closely to a regular 

 octahedron truncated by a cube in such a way that all the 36 

 edges of the resulting figure are of equal length. Of the 14 

 faces, the 6 corresponding to the cube are true planes, whereas 

 the 8 corresponding to the octahedron are slightly curved. 

 The curvature of these faces was found approximately by Lord 

 Kelvin, but the exact expression for these surfaces appears to 

 be as yet unknown. 



The assumption of Cauchy and Poisson which has led to so 

 much controversy between the uniconstant and biconstant theo- 

 ries of isotropy, was merely that molecules act as mass points, 

 attracting or repelling from their centroids. This was a very 

 natural assumption, and, as Saint Venant pointed out, is no 

 other than that made by Newton in developing the theor}' of 

 gravitation, viz., that celestial bodies attract towards their cen- 

 ters. It is also known that some substances, especially glasses, 

 nearl}' fulfill the conditions expected by Cauchy and Poisson, 

 that is, <T equals nearly ^. On the other hand, the experi- 

 ments of various physicists, and especially of Voigt, show that, 

 for crystalline substances, the rariconstant theory of elasticity 

 is totally untenable and a often differs greatly from \. Now, it 

 seems pertinent to reflect that while from certain points of view 

 the planetary masses may be regarded as mass points, when 

 phenomena such as that of precession and mutation are con- 



