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Proceedings of Poyal Society of Edinburgh. [sess. 
atoms of small surface and close in the grain will not unite so 
closely, and thus may leave a greater hold for their molecules to 
unite. There are some atoms, I take it, so close-grained and 
small in surface proportional to weight as not even to form 
molecules before they solidify. For instance, mercury. Mercury 
is mon-atomic, and the above, I believe, is the reason for this 
fact. There are some others, though, which, after the union of 
the atoms, leave so great a hold for the attachment of their 
molecules, by reason of their great squeezability, that they have 
not yet been liquefied, so far as I know, much less vaporised. 
For instance, carbon. 
It is for mathematicians and physicists to apply this principle 
to particular cases, for I confess that I myself, being no mathe- 
matician, am unable to do so. 
Chemical Affinity. 
Chemical affinity appears to be caused by the differences in the 
sizes and closed-grainedness and generally the squeezability of atoms. 
An atom of one kind may find a better hold on an atom of another 
kind than on one of its own kind. For instance, take two spheres, 
a large one and a small one, and press the two together, and if they 
are at all compressible, the area in contact with the small sphere 
will be larger than if it were pressed against a sphere of its own 
size. Again, take two spheres of the same size, but one being more 
compressible than the other, and press them together. The least 
compressible one would be in contact with a greater surface area 
than if it were pressed with the same force against another of its 
own kind. This, then, appears to me to be the principle governing 
chemical affinity. Small atoms, finding a better hold on large 
atoms than on their companions, quit the latter, and attach them- 
selves as parasites to the former, crowding the large ones out by 
taking up the available amount of grip, and so causing the large 
ones to lose their hold on one another ; or, a less squeezable atom 
finding a better hold on a more squeezable one, quits its companion 
for the latter. Of course it is only when circumstances are 
favourable that the change takes place, such as when the elements 
are in a state of solution, or when in the gaseous state, or in the- 
nascent state; or sometimes a gas unites with a solid, as when 
