424 TRANSACTIONS OF SECTION B, 
consequences during the period under review, the completeness with which 
the fundamental properties of the carbon atom are symbolised by a regular 
tetrahedron being altogether astounding. 
Our present conception is that the carbon atom has tetrahedral properties 
in the sense that it has four affinities which operate practically in the 
direction of the four radii proceeding from the centre towards the four 
solid angles of a regular tetrahedron. 
More than analogical significance—to use Larmor’s expression—must 
be accorded to this symbol on account of its remarkable accordance with 
the facts generally, whether derived from the study of asymmetric optically 
active substances or from observation of the activity of ring structures of 
various degrees of complexity. Nothing is more surprising than the com- 
pleteness with which the vast array of facts included in organic chemistry 
may be ordered by reference to the tetrahedral model. In the future, when 
our civilisation is gone the way of all civilisations and strangers dig on 
the sites of our ruined cities for signs of our life, they will find the 
tetrahedron and the benzene hexagon among the mystic symbols which they 
have difficulty in interpreting ; if, like the ancient Egyptians, we made our 
tombs records of our wisdom, such symbols would long since have acquired 
sacred significance and probably the public would have learnt to regard them 
with awe and to respect them as totems. Chemists might at least wear them 
on aprons in imitation of the Freemasons; perhaps no two other symbols 
have so great a significance—they reach into life itself. 
It would seem that carbon has properties which are altogether special, 
the influence which it exercises upon other elements in depriving them of 
their activity is so remarkable. In their recent discussion of the relation 
of crystalline form to structure, in which valency is represented as a 
function of the volume sphere of influence exercised by an element, Barlow 
and Pope arrive at the remarkable conclusion that carbon is probably the 
only element the atom of which has a volume sphere of influence four 
times that of the hydrogen atom; although it combines with four atoms 
of hydrogen, silicon apparently has only half the volume sphere of influence 
of carbon. This may, in a measure, account for the very great dissimi- 
larity in behaviour of the two elements, which is most pronounced in their 
oxides, the single atom of carbon all but dominating two atoms of oxygen 
in carbon dioxide (which is consequently gaseous), whilst the atom of 
silicon in silicon dioxide in no way eclipses the two atoms with which it is 
associated but leaves both charged with residual affinity which enables 
them to form complex collocations of remarkable fixity in the fire. At 
bottom the differences between organic and inorganic nature are to be 
regarded as very largely the expression of this difference. Ropes of sand 
are proverbially treacherous: yet without sand, if silica had been a gaseous 
substance, our world might have worn a strangely different aspect.’ 
1The solid model of silica which Barlow and Pope have constructed has 
very remarkable attributes, in that the oxygen atoms appear to be uniformly 
related and in intercommunication throughout its mass : so that a mass of silica, 
whatever its size, may almost be regarded as a single molecular complex. <A 
similar view may be taken of plastic metals such as those of the platinum group, 
gold, silver and copper. Whether when rendered brittle by association with 
small amounts of impurity these are resolved into simpler molecular complexes 
or whether the molecules merely become separated by substances which promote 
discontinuity and brittleness, it is impossible to say at present. The cauge of 
hardness in mineral materials is, however, a question of no slight interest and 
importance. The property is strikingly exemplified in the diamond. It is 
difficult to understand the intense hardness of this material, on the assumption 
that the diamond is composed of paraffinoid carbon—that is to say, carbon with 
al] its affinities satisfied. At present we appear to have no clue to the manner 
in which affinity acts in promoting the formation of such solids. But it is 
obvious that all solids are possessed of some degree of ‘surface affinity,’ as 
