52 SECTIONAL ADDRESSES. 
oxygen atoms are grouped in some sort of tetrahedral fashion about the 
sulphur atom. 
There are still other connections between structure and properties 
which we begin to understand, and can use in proportion to our under- 
standing. The cleavage plane, and the occurrence of certain faces in 
preference to others are connected with the nature of the bonds and the 
size of the spacings. We are not surprised to find that in bismuth, or 
graphite or naphthalene, the cleavage plane cuts across the ties which we 
should expect to be the weakest of those that bind the molecules together ; 
or again, that natural faces follow the planes that are richest in atoms 
or molecules and may be assumed to contain relatively large numbers of 
linkages. In naphthalene the cleavage plane passes between the ends of 
the molecules, where the 8 hydrogens are, and where there is a deficiency 
in the number of scattering centres, as the X-rays indicate by the strengths 
of several orders of the (001) reflection. The other faces found on the 
crystal cut across the ties at the positions of the « hydrogens. 
There are many other connections between the structure and other 
properties of a substance, such as dielectric capacity, rigidity, and compres- 
sibility, conductivity both thermal and electric, magnetic constants. In 
fact, the only properties of solid bodies which are not directly and obviously 
related to crystal structure are those, few in number, that depend on atomic 
characteristics alone, such as weight; and the absorption coefficients for 
#, 8, y and X rays, all the rays which involve high quantum energies. 
With few exceptions every aspect of the behaviour of a solid substance 
depends on the mode of arrangement of its atoms and molecules. We 
have, therefore, an immense field of research before us, into which the X-ray 
methods have provided an unexpected and welcome entrance. 
They tell us directly, as I have said, the number of molecules in the 
crystal unit cell, and the mode of their arrangement with such determina- 
tion of lengths and angles as are required to define the mode of arrange- 
ment in full. They leave us then to ally our new knowledge to all that we 
possess already as to the physical and chemical properties of substances. 
By this comparison we hope in the end to determine the position of every 
atom, and explain its influence through its nature and position upon the 
properties of the substance. It is the chemistry of the solid that comes into 
view, richer in its variety even than the chemistry we have studied for the 
past century, and possessing an importance which is obvious to us all. 
Every side of scientific activity takes part in this advance, for all sciences 
are concerned with the behaviour of matter. 
