1888.] Prof. Liveing, On Solution and Crystallization. 221 
of all cavities—the segmentation cavity—as the only system of 
spaces between the endoderm and ectoderm: whilst the primitive 
segmentation cavity has differentiated in the higher animals, on 
the one hand into body-cavity—Annelids, and on the other in 
the cavities of the vascular system—Vertebrates. 
(2) On the Secretion of Salts in Saliva. By J. N. LANGLEY, 
M.A., Trinity College, and H. M. Fiercusr, B.A., Trinity College. 
May 21, 1888. 
Mr J. W. CLARK, PRESIDENT, IN THE CHAIR. 
The following communications were made: 
(1) On Solution and Crystallization. By Professor LIVEING. 
[ Abstract. | 
When a substance passes from a state of solution into the 
solid state, the new arrangement of the matter must be such 
that the entropy of the system is a maximum; and, other things 
being the same, the surface energy of the newly formed solid must 
be a minimum. If the surface tension be positive, that is tend 
to contract the surface, the surface energy will be a minimum when 
the approximation of the molecules of the surface is a maximum. 
The essential difference between a solid and a fluid is that the 
molecules of the former maintain approximately the same relative 
places whereas the molecules of a fluid are subject to diffusion. 
Further, crystalloids in assuming the solid form assume a regular 
arrangement of their molecules throughout their mass, which we 
can usually recognise by the optical properties of the crystal, and 
by the cleavage. If we suppose space to be divided into cubes by 
three sets of parallel planes, each set at right angles to the other 
two, and suppose a molecule to be placed in every point where 
three planes intersect, we shall have an arrangement which cor- 
responds with the isotropic character of a crystal of the cubic 
system. But of all the surfaces which can be drawn through 
the system the planes bounding the cubes meet the greatest 
number of molecules, those parallel to the faces of the dodeea- 
hedron meet the next greatest number of molecules, and those 
parallel to the faces of the octahedron meet the next greatest 
number. Also if we take an angular point of one of the cubes 
as origin, and the other three edges of the cube as axes, and the 
length of an edge of the cube as the unit of length, every plane 
16—2 
