438 TRANSACTIONS OF SECTION B. 
solutions, however inadequately, I must now endeavour to justify my open- 
ing reference to the importance of the organic side of our science. 
The province of organic chemistry is so vast that it may appear to be 
difficult to distinguish the main lines of advance from the by-paths which 
intersect the field of inquiry in every direction. In reality this is not 
the case; certain salient features stand out which must attract attention 
if once attention be drawn to them. The efforts of the chemist to elucidate 
structure and to correlate structure with function have been extraordinarily 
successful. In the first place, as already remarked, the student of the 
subject now has his attention concentrated on the tetrahedron as the symbol 
of the functional activity of carbon ; however numerous the compounds, he 
knows that certain simple rules can be laid down as applicable to all. It 
is established beyond question that carbon atoms have a remarkable ten- 
dency to form ring systems. The affinities of the atom seem to act almost 
rigidly in certain directions, which appear to be those of lines drawn from 
the middle point of a regular tetrahedron to its angular points. Rings 
containing either five or six atoms of carbon are therefore those which are 
most readily formed and of maximum stability ; carbon atoms may and do 
unite in pairs, threes, and fours, but compounds of this order are far less 
permanent than those containing either five or six atoms, as the affinities 
appear to meet in such a manner that they do not satisfy each other and 
consequently the compounds enter somewhat readily into combination with 
other substances. When the number of carbon atoms exceeds six, not only 
is there less tendency to form a ring system but the stability of the system 
is slight ; when the number is considerable, stability is attained by the for- 
mation of complex systems consisting of several rings conjoined (camphor, 
naphthalene, anthracene, etc.). 
The behaviour of carbon compounds generally, in so far as this may be 
regarded as dependent on the condition of the carbon, is extraordinarily 
simple and may be summed up in the statement that it is either paraffinoid, 
ethenoid, or benzenoid.* Paraffinoid carbon is incapable of combining with 
other substances and but slightly attractive, so that the hydrogen atoms are 
by no means easily displaced from paraffinoid compounds ;? ethenoid carbon 
combines readily with various substances, forming compounds of paraffinoid 
type; in the benzenoid state carbon appears to combine somewhat readily 
with a variety of substances, but the products enjoy only an ephemeral 
existence and usually escape notice, as they at once break down, giving 
rise to benzenoid substitution derivatives, so that in this last form carbon 
simulates the paraffinoid state but is more active. 
In earlier years our attention was concentrated on benzene and the 
benzenoid compounds; much was done to elucidate the structure of these 
hydrocarbons and of their derivatives ; meanwhile these latter have proved 
to be of extraordinary significance technically, notably as dye-stuffs, but 
also on account of their medicinal value, as perfumes, and in photography. 
The structure of benzene has been the subject of much discussion during 
the period under consideration. I trust I shall not be accused of parental 
bias if I urge that the centric’ formula is the best expression of the 
* A fourth condition requiring recognition is that of the carbon in acetylene ; 
at present the acetylene compounds are so few in number, however, that this form 
may be left out of account. 
* The displacement of the hydrogen aesociated with carbon is in all probability 
a secondary phenomenon; it is likely that this is true generally and that hydrogen 
is never merely remoyed or attracted away but always has its place taken by a 
radicle which becomes temporarily attached to the multivalent atom with which 
the hydrogen is associated. 
* I have discussed this matter somewhat fully in a recent essay, with reference 
to the nature of amorphous carbon, in connexion with the remarkable work of 
Sir James Dewar on the absorption of gases by charcoal at low temperatures 
(Journal of the Royal Institution). 
