discourse delivered in the Royal Institution of
Great Britain, June 6, 1856.]

WHEN the student of physical science has to investigate the character
of any natural force, his first care must be to purify it from the
mixture of other forces, and thus study its simple action. If, for
example, he wishes to know how a mass of liquid would shape itself if
at liberty to follow the bent of its own molecular forces, he must see
that these forces have free and undisturbed exercise. We might
perhaps refer him to the dewdrop for a solution of the question; but
here we have to do, not only with the action of the molecules of the
liquid upon each other, but also with the action of gravity upon the
mass, which pulls the drop downwards and elongates it. If he would
examine the problem in its purity, he must do as Plateau has done,
detach the liquid mass from the action of gravity; he would then find
the shape to be a perfect sphere. Natural processes come to us in a
mixed manner, and to the uninstructed mind are a mass of
unintelligible confusion. Suppose half-a-dozen of the best musical
performers to be placed in the same room, each playing his own
instrument to perfection, but no two playing the same tune; though
each individual instrument might be a source of perfect music, still
the mixture of all would produce mere noise.

Thus it is with the processes of nature, where mechanical and
molecular laws intermingle and create apparent confusion. Their
mixture constitutes what may be called the _noise_ of natural laws, and
it is the vocation of the man of science to resolve this noise into
its components, and thus to detect the underlying music.

The necessity of this detachment of one force from all other forces is
nowhere more strikingly exhibited than in the phenomena of
crystallisation. Here, for example, is a solution of common sulphate
of soda or Glauber salt. Looking into it mentally, we see the
molecules of that liquid, like disciplined squadrons under a governing
eye, arranging themselves into battalions, gathering round distinct
centres, and forming themselves into solid masses, which after a time
assume the visible shape of the crystal now held in my hand. I may,
like an ignorant meddler wishing to hasten matters, introduce
confusion into this order. This may be done by plunging a glass rod
into the vessel; the consequent action is not the pure expression of
the crystalline forces; the molecules rush together wi