ances is illustrated by the well-known fact that water is an
almost universal solvent. It is its residual affinity which enables it
to enter into weak chemical combination with a large number of other
substances, and thus to dissolve those substances. The dissolving power
usually increases when the temperature is raised, possibly because the
self-contained or self-sufficient groupings of the water molecules are
then to some extent broken up and the fragments enabled to cling on to
the foreign or introduced matter instead of only to each other. The
foreign substance is apt to be extruded again when the liquid cools,
and when the affinity of the water-aggregates for each other resumes
its sway. Very hot water can dissolve not only the substances
familiarly known to be soluble in water, but it can dissolve things
like glass also; so that glass vessels are unable to retain water kept
under high pressure at a very high temperature, approaching a red heat.

Another material which also seems to have the power of combining with a
number of other bodies, under the influence of the loose mode of
chemical combination spoken of as residual affinity, is carbon; so that
a block of charcoal can absorb hundreds of times its own bulk of
certain gases.

Indeed, Sir James Dewar has recently employed this absorbing power of
very cold carbon to produce a perfect kind of vacuum, which may,
perhaps, be the nearest approach to absolute vacuum that has yet been
attained: probably higher than can be attained by any kind of
mechanical or mercury pump.

_Unexpected Influence of Size._

Suppose now a substance contains a great number of carbon molecules and
a great number of water molecules, each of which has this residual
affinity or power of clinging together well developed, what may be
expected to be the result? Surely, the formation of a molecule
consisting of thousands or hundreds of thousands of atoms, constituting
substances more complex even than those already known to or analysable
by organic chemistry; and if these complex molecules likewise possess
the adhesive faculty, a grouping of millions or even billions of atoms
may ultimately be formed. (A billion, that is a million millions, of
atoms is truly an immense number, but the resulting aggregate is still
excessively minute. A portion of substance consisting of a billion
atoms is only barely visible with the highest power of a microscope;
and a speck or granule, in order to be visi