185 
the single compound which it forms with chlorine, is assumed 
as the standard of this force (I was about to say). I do not 
like the term “force,” however, as it scarcely seems to describe 
accurately a power acting ab intra and not ab extra ; but this 
former power is what very specially characterises the properties 
of matter, as seen from a chemical point of view. 
55. We must, in fact, regard atoms as “ substantial ” centres 
of force (if this term be used), and as combining with mathe- 
matical certainty on the lines of their affinities, or separating, 
as the result of superior attraction on the one hand, or of 
force (ab extra) on the other. 
56. I may be pardoned for explaining my meaning by 
reference to familiar use of words. When two young persons 
have a liking for each other, we do not call this force; and yet 
their caring for each other may have very important influence 
on their future destinies. Now we are compelled to speak of 
the affinities of atoms as very important indeed, and these 
“ affinities ” if “unsatisfied” may lead to the dismemberment 
of the molecule ; or, shall we say, to the breaking up of the 
household. In some cases two individual atoms are quite 
taken up with each other, as chlorine and hydrogen ; and are 
consequently termed monatomic, monadic, or univalent. But 
oxygen unites with two atoms of hydrogen, and is diatomic, 
dyadic, or bivalent. Nitrogen combines with three atoms of 
hydrogen, carbon combines with four atoms of hydrogen, and 
so forth. 
57. It is fortunate that we are able to calculate with perfect 
certainty on the basis of the permanent likings or dislikings 
of these small bodies. Though violence may sever, they 
always retain the same measure of affection.* It is equally 
fortunate that when a molecule is established by means of 
these affinities, there is no law to render the alliance stable. 
Chemical change mostly depends on our being able to entice 
away an atom from its molecule, i or to present to a molecule 
that has taken possession of more atoms than it can well keep 
a more tempting object which replaces another in the magic 
ring. This is called, in chemical language, substitution. 
58. Thus chlorine and bromine may in many instances be 
introduced into hydrogen compounds by direct substitution ; 
* Of course, I use the language of metaphor. I have not the least 
approach to a conception of the hoiu or the why of these affinities. The 
suggested “ harmony of molecular movements ” are to me like the Mediaeval 
explanations of the movements of the heavenly bodies, — “ Cycle on cycle, 
orb in orb,” nor is “ the impact of atoms of luminiferous ether on opposite 
sides ” more explanatory. 
