PRESIDENTS ADDRESS—SECTION B. D7 
Further light was thrown on the constitution (apart from 
theoretical considerations) of organic compounds by the dis- 
coveries of the radicle ethyl by Frankland (1848), of compound 
ammonias by Wurtz (1849), and of the constitution of ether 
by Williamson, and many others. 
In 1852 a new direction was given to our views respecting the 
linking of atoms by Frankland’s discovery of zinc ethyl, and a 
series of similarly constituted organo-metallic compounds. This 
led Frankland to the recognition of the fact that each atom has 
a definite combining power, a definite valency, and that this 
valency is possessed also by groups of elements or radicles.  I[t is 
the valency of the elements that determines their combination 
and the constitution of the compounds formed. 
The peculiar manner in which the carbon atom links itself to 
other carbon atoms was particularly studied by Kekulé (1858), 
to whom we owe our present lucid ideas as to the structure of 
carbon compounds, and who may be said to have founded that 
enormous field of organic chemistry, which deals with the 
Renzene derivatives by his explanation of the constitution of 
Benzene. 
The most recent theoretical considerations of the linking of 
the caybon atoms has been the conception of geometrical struc- 
tural formule in space of three dimensions, the introduction of 
which we owe to Van’t Hoff and Le Bel, by means of which a 
beautiful and satisfactory explanation of the constitution of sub- 
stances of different rotating power has been possible. 
ELECTRO-CHEMICAL THEORY. 
We have seen already that the idea of electricity and chemical 
affinity being identical was held in the time of Berzelius. 
Davy had already expressed himself convinced of the identity 
of the two forces. In 1838 Faraday enunciated the law of 
electrolytic equivalents already alluded to. 
Before we are in a position to state exactly what happens 
when a current of electricity is passed through an electrolyte we 
have first to learn something more about the nature of sub- 
stances in solution. 
In 1857 Clausius showed that the current of electricity was 
not the cause of the decomposition, but that the action of elec- 
tricity was to effect a separation of the already dissociated ions. 
Raoult, in 1883, showed that the depression of the freezing 
point of a liquid by the solution in it of another liquid or a 
solid was proportional to the amount of the dissolved substance, 
and inversely proportional to its molecular weight. This law is 
of special importance in determining the molecular weight of 
those substances whose vapour density cannot be taken, and to 
which consequently Avogadro’s law is inapplicable. Raoult also 
