Strong spirit, or =alcohol=, is a clear transparent fluid which looks
like water, but is a very different substance. For example, it boils at
a much lower temperature, it burns with a blue flame, it has
intoxicating properties, and, like oil, it is very much lighter than
water. Hence, if coloured spirit is poured gently upon the surface of
water the spirit rests upon the water. Suppose, now, that we take a tall
measure graduated into ten equal parts. Fill the lower five with water,
and then, very gently, pour in the strongest alcohol, coloured in some
way, until the tenth mark is reached. We shall have five volumes of
water below, and an equal quantity, or five volumes, of coloured alcohol
above. Where the two are in contact, the colour will be diffused into
the water for a short distance, but not far, showing that only a slight
mixture is taking place. This, however, is not because the two fluids
mingle with difficulty; for, with slight stirring, they mix completely,
and you have a fluid the colour of which is about half as intense as
that of the alcohol, and many of the other properties of which are
intermediate between those of pure alcohol and those of pure water.
Thus far, nothing further than simple mixture, as when coloured water
was added to pure water, seems to have occurred; but, in reality,
something more has happened. In the first place, the mixture is a good
deal warmer than either of its components; that is to say, =heat= has
been =generated=. In the second place, if you measure the volume of the
whole fluid after it has cooled, it no longer stands at the mark =ten=,
but distinctly lower, or about =nine and three-quarters=. As the volume
of the mixture is less than the sum of the volumes of its two
components, it follows that the =density= of the mixture must be
=greater= than a density midway between that of the water and that of
the alcohol. In other words, the molecules in the mixture do not occupy
the same space as they did when they were separate. The result is the
same as if the ten volumes had been compressed until they occupied only
nine and three-quarters; so that the effect is a contraction similar to
that which would be brought about by taking away heat from the mixture.
In fact, as we have seen, the mixture gives out a quantity of heat.
There is another respect in which the mixture is unlike both its
constituents. It both =boils= and =freezes= at a much =lower=
temperature than =water= does, and at a =higher= temperature than
=alcohol= does. In fact pure alcohol has not yet been frozen. If the
molecules of the alcohol were merely diffused among those of the water
as water is diffused through wet sand, they ought to pass into the
gaseous state at the same temperature as that at which alcohol boils;
and it would then be very easy to separate alcohol from water by
distillation. But the fact is not so; alcohol cannot be obtained free
from water by distillation unless something which holds water very
strongly, such as quicklime, is added, so as to keep all the water back
when the fluid is heated.
Thus alcohol and water, mingled together, give rise to a fluid which is
not a mere mixture, the properties of which are known if we know the
properties of its components; it is, in strictness, a new body, in which
the molecules of the water and those of the alcohol affect one another
to a certain extent and modify the pre-existing properties of each.
This effect of different bodies upon one another becomes much more
manifest when water is brought into contact with certain solids.