50 SECTIONAL ADDRESSES. 
In calcite the three oxygens which surround a carbon atom must lie 
in one plane. It is supposed, however, that in this case the bonds are 
electrostatic : the carbon atom has lost its four valency electrons, and with 
them its powers of tetrahedral orientation. 
Now if we can discover the extent to which an orientation is maintained 
under different conditions we are provided with one more guiding principle 
in our attempt to discover the structure of the crystal which contains carbon 
atoms. And, of course, the organic compounds centre round the carbon 
atom and its tetrahedral structure. 
The question of orientation in respect to other atoms is more obscure, 
but it is clearly one of importance. There must be some reason why ice 
has such an open structure, and here the oxygen atom is largely concerned. 
In the ruby the oxygen atom has no plane of symmetry in relation to its 
neighbours. In organic substances the great emptiness of the structure 
implies that atoms are attached to one another at points which have definite 
positions on the surfaces of the atoms and are limited in number. And, 
generally speaking, the consideration of organic crystal structure is against 
any idea that atoms and molecules are to be treated as spheres surrounded 
by uniform fields of electric force, except in certain cases where by loss or 
gain of electrons an atom has been reduced to the outer form of one of the 
rare gases. They must have highly irregular fields, having forms which 
more or less resist any change. The weak bonds which hold molecule to 
molecule in the organic substance are not due to electron sharing as in 
diamond, or to ionisation as in rock-salt, but to an intermingling of stray 
fields belonging to definite positions on the surfaces of the molecules. 
Our attempt to discover the effect of orientation is part of a general 
attempt to discover the field of force of the atom, which is naturally a very 
difficult matter. But if we can learn only a few rules, even empirical 
rules, we areso much the further on our way. 
Yet another obvious and most important source from which help may 
be obtained is to be found in chemistry itself. Although the chemist 
has had no means until now of measuring distances and angles, he has 
been able to build up a wonderful edifice of position chemistry. Anatom 
A of a molecule is certainly linked, it may be to B, and not to C; or again, 
of a number of atoms of the same nature and contained in the same mole- 
cule, so many must be alike, and so many may be different. 
The chemist has, for example, come to the conclusion that the naphtha- 
lene molecule is a double benzene ring, and the anthracene a triple benzene 
ring. The X-ray observations show that one of the sides of the unit cell 
of the latter crystal is longer by 2.5 A.U. than the corresponding side of 
the other, all other dimensions of the two cells being very nearly the same. 
The width of the hexagonal ring in the diamond is 2.5 A.U., so that on the 
one hand the chemical evidence suggests that the length of the molecule is 
parallel to that edge of the two cells which shows differing values, and on 
the other the X-ray conclusions give material support to the chemical view. 
Let us take another example from basic beryllium acetate, Be,O(C,H,0,),. 
The substance is remarkable for the ease with which it sublimes into 
a vapour consisting of whole molecules, from which we may infer 
that the molecule does not suffer much change in the process. The 
relative positions and mutual alliances of the atoms are nearly the same 
when the molecule is free as when it is built into the solid, From the 
