PRESIDENT’S ADDRESS—-SECTION A. 33 
by Frankland (z) of the theory of the saturation-capacity, or 
valency of chemical elements. The complement of this brilliant 
addition to the atomic idea was the conception of the hnking or 
union of atoms (y), for which we are indebted to Kekulé and 
Couper in 1858. 
An excellent way of realising what the atomic and molecular 
conceptions mean to chemistry is to consider the steps by which 
the empirical, molecular, rational, general, constitutional, 
eraphic or structural, projective, and stereometric formule of a 
substance are established (z). It is easy to see that the degree 
of difficulty continually increases; a nicer appreciation of evi- 
dence is required for the higher formule; the equipment in 
knowledge and apparatus becomes more extensive, and indeed the 
boundary between chemistry and physics here disappears. 
30. Molecular Arrangement and Physical Properties—Time 
will not admit of an historical reference to those labours of 
genius by which the connection between chemical constitution 
and physical preperties has been wholly or partly identified. To 
some extent the relations to the density, viscosity, to the light 
refractive, and rotatory power of substances have been ascer- 
tained. Within certain limits it has been conclusively proved 
that the melting and boiling points of some series of chemical 
compounds depend solely upon the minimum and maximum 
moments of inertia of their molecules rotating about their centre 
of figure. The heat of combustion of hydrocarbons has been 
shown to throw light upon the nature of their atomic lnkage. 
The general application of the principles of thermodynamics to 
chemistry is revealing more deeply the exact nature of chemical 
changes. The kinetic theory of gases has proved of utility in 
explaining the phenomena of osmotic pressure; in other words, 
in dilute solutions the molecules behave as though in the gaseous 
state. The instability of certain compounds, and the nature of 
explosive substances, is much better understood than formerly, 
as also the relations of temperature to the velocity with which 
substances react. Dissociation by temperature, or electric pres- 
sure, has been made intelligible, and a definite theory of electro- 
chemistry established. The facts of crystallisation are also being 
brought within the domain of mechanical interpretation. The 
sudden changes of form, seen sometimes in crystallising bodies, 
are conceived to be due to rearrangement under molecular 
stresses, so that the system of packing of the molecules shall be 
(x) [Born 1825. } 
(y) Verkettung oder Bindung der Atome. 
(z) The first affirms composition only; the second, the number of atoms in the mole- 
cule; the third, its general chemical character; the fourth, the class of compounds to 
which it belongs; _the fifth epitomises its chemical character in full; the six or seventh 
represents that epitome by showing conventionally the position of each atom; the eighth 
and ninth, the space relations of the components of the molecule—the former conyen- 
tionally, the latter perspectively. 
Cc 
