56 Proceedings of Royal Society of Edinburgh. |_ SESS - 
are closely united, or the reverse, — a fact which can he tested by 
their melting-points, as to whether they may he good or had 
conductors. — for molecules with a small hold on one another cannot 
transmit their motion from one to another so well as those which 
are closely united ; and, moreover, the amplitude of vibration would 
be greater with the former, thus increasing the radiation. 
There are two classes of bodies which most strongly differentiate 
themselves one from the other by being good conductors as a class 
or bad conductors as a class. These are the metals and the non- 
metals. The metals are all good conductors, though they differ 
greatly amongst themselves, while the non-metals are nearly all 
bad conductors. The metallic atoms, then, must be all close- 
grained, and this would seem to account for their great specific 
gravities, while the non-metallic atoms must be all open-grained, 
which would account for their low specific gravities. 
Now, it is found that different bodies unite with one another in 
different proportions, and this is termed “valency.” To account 
for this in full is most difficult. But I believe that the cause is the 
different squeezabilities of atoms. For instance, two monads unite 
with a dyad because the dyad is squeezable to the extent of afford- 
ing a sufficient hold to two bodies of the monad class. Three 
monads unite with a triad because it is squeezable to the extent of 
affording a hold to three bodies of the monad class, and so on. The 
higher the valency, the greater the squeezability of the nucleus of the 
resulting molecule. Now, metals are amongst the atoms of highest 
valency; and this would seem to contradict what I have said about 
metals as a class being the least squeezable. But we must 
remember that metals resist compression because they are more or 
less near the point of absolute unsqeezability, but some of them may 
be so far from this point as to admit of a considerable amount of 
compression, more especially when we regard their generally high 
atomic weights. On the other hand, non-metals, although their 
absolute squeezabilities are far greater than metals, may practically be 
far less squeezable, having regard to their generally less atomic 
weights, owing to the greater velocity of their particles. To illus- 
trate this point, and give the reader a clear idea of my meaning, let 
us suppose an atom to be so close-grained that no compression of it 
was possible. Such an atom would have no valency ! simply 
because no other could stick to it if that other were as unyielding 
