PRESIDENTIAL ADDRESS. 327 
of Organic Chemistry by methods which have now become traditional; progress 
has been made by the application of strictly logical methods of interpretation 
to masses of experimental data, and each new conclusion has been checked and 
verified by the accumulation of fresh contributions in the laboratory. The 
sureness of the methods adopted could not fail to lead to the intrusion of 
stereochemistry into adjacent fields of scientific activity; bio-chemistry, the 
study of the chemical processes occurring in living organisms, is already largely 
dominated by stereochemistry, and the certainty with which stereochemistry has 
inspired us as to the reality of the molecular constitution of matter is exerting 
a powerful influence in other branches of natural science. Quite possibly, 
however, the acquaintance which every chemist possesses of the great progress 
already made upon one particular set of lines is to some extent an obstacle 
to his appreciation of new directions in which further great stereochemical 
advances may be anticipated. 
A little reflection will show that the study of the relation between the 
crystalline form and chemical constitution or configuration of substances in 
general may confidently be expected to lead to important extensions of our 
knowledge of the manner in which the atoms are arranged in molecular com- 
plexes. The earlier crystallographic work of the nineteenth century led to the 
conclusion that each substance affects some particular crystalline form, that the 
regular external crystalline shape is an expression of the internal structure of 
the crystal, and that a determination of the simpler properties—geometrical, 
optical, and the like—of a crystalline material constitutes a mode of completely 
characterising the substance. Later work during the last century demonstrated 
that the properties of crystalline substances are in entire harmony with a simple 
assumption as to the manner in which the units or particles of the material 
are arranged; the assumption is that the arrangement is a geometrically 
‘homogeneous’ one, namely, an arrangement in which similar units are uni- 
formly repeated throughout the structure, corresponding points presenting every- 
where a similar environment. The assumption of homogeneity of structure 
imposes a definite limitation upon the kinds of arrangement which are possible 
in crystals: it leads to the inquiry as to how many types of homogeneous 
arrangement of points in space are possible, and to the identification of these 
types with the known classes of crystal symmetry. The final conclusion has 
been attained that there are 230 geometrically homogeneous modes of distributing 
units, or points representing material particles, throughout space; these, the 
so-called 230 homogeneous ‘point-systems,’ fall into the thirty-two types of 
symmetry exhibited by crystalline solids. The solution of the purely geometrical 
problem here involved was commenced by Frankenheim in 1830, and finally 
completed by Barlow in 1894; it brings us face to face with the much larger 
stereochemical problem—that of determining what the units are which become 
homogeneously arranged in the crystal, why they become so arranged, and in 
what way a connection can be established between chemical constitution and 
crystal structure. 
Since the conception of homogeneity of structure alone is clearly insufficient 
for the interpretation of the more advanced problem, some further assumption 
must be made as a foundation for any really comprehensive attempt to collate 
the quantities of isolated facts bearing upon the subject. Of the many assump- 
tions which have been made in this connection only one, which may now be 
stated, has as yet proved fruitful in the sense that it serves to correlate large 
numbers of known experimental facts, and that it indicates the way to the 
discovery of fresh facts. The assumption is that each atom in a crystalline 
structure acts as a centre of operation of two opposing forces: (a) a repellent 
force, attributable to the kinetic energy of the atom, and (b) an attractive 
force, both forces, like gravity, being governed by some inverse distance law. 
Such an assumption forms an essential part of the classical work of Clerk 
Maxwell and van der Waals on the kinetic theory of gases and liquids. Its 
application to solid crystalline substances, where it must be applied in con- 
junction with the principle of structural homogeneity, was made by Barlow 
and myself in 1906. 
The operation of the assumption just stated is readily visualised by con- 
sidering the simplest possible case, that, namely, of a crystalline element each 
