which changed circumstances produce in the mineral regions, and occasion
a change of that rule of action which we have learned from experience,
when melting and evaporating those substances in the atmosphere or on
the surface of the earth.

It is in this manner that a theory, which was formed by the
generalization of particular facts, comes to be a source of information,
by explaining to us certain appearances which otherwise we could not
understand. Thus, it was not the appearance of the tides that taught the
theory of gravitation; it was the theory of gravitation that made us
understand the appearance of the tides. In like manner, the law of
gravitation, which was demonstrated from the motion of the moon in her
orbit round this earth, when applied to the paths of comets, explained
that appearance. Our theory, of a central fire, has been formed upon
the consolidation of the strata of this earth; but this theory is to be
applied for the explanation of various different appearances. In this
manner, two different purposes will be served; the trying of the theory
by its application to phenomena; and the explanation of phenomena by the
principles laid open in the theory.

I may repeat it; a theory of the earth must ultimately depend upon
matter of fact or particular observation; but those observations must be
distinct, and those distinguished things must be generalised. We have
just now given for an example, a distinction among stones, in knowing
them by their sensible qualities. But, besides distinguishing those
objects, we are also to inquire into the origin and cause of those
things which are distinguished. Here, again, we take into our aid the
chemical as well as the mechanical properties of these several things;
and hence learn to know on what their natural form and constitution may
depend. Having thus attained the natural philosophy of stones, we next
inquire into the place and application of those things in nature; and
in this manner we acquire some knowledge with regard to the natural
constitution of this earth. We find this earth composed of known things;
it is therefore the operations, required in these compositions, which
form the natural philosophy of this earth, considered as a body of solid
land. But, the solid land is only one part of the globe; therefore,
the philosophy of the globe proceeds still farther by knowing the
constitution of this planetary body, as consisting of different parts
united for a purpo