438 J. W. Langley—Chemical Affinity. 
atomic interchange; hence that affinity is fundamentally due 
ation. 
We see that the primitive notions of affinity have undergone 
extensive modifications. dea which seemed so simple 
and natural a one to Hippocrates has grown successively more 
complex and less sharply defined; for while it presented itself 
to him as a single cause depending on kinship or occult resem- 
blance we now find it branching out into a many stemmed 
structure inextricably entangled with other physical forces and 
having its roots deep down in the regions of the mysterious and 
the unknown. When we look for an advance in precision of 
ideas, for a logical development of a satisfactory theory, or for 
generalizations which shall help us better to classify chemical 
henomena in terms of force and energy, we are compelled to 
admit that the years have not brought the theory of affinity to 
a state of active growth; rather it is like that strange counter- 
part of a living tree, the branching coral, whose many busy 
workers do indeed each for themselves add their mites to the 
accretions of past generations, but who have failed with all 
their toil to infuse that mysterious principle which would make 
of their labors a living organism ruled by an internal law of 
growth. 
Affinity, under its own name, is no longer presented in 
recent manuals. Chemists have more and more turned their 
and thus they lose the great advantage of being bound together 
under one title. 
Secondly, there is a more important reason arising from 
what has hitherto been the traditional scope of our science. 
