Myriads of shells, bones and plants scattered through the earth’s strata have been transformed in the manner indicated. Although petrifaction is usually a long process, there is reason to believe that it sometimes takes place rapidly. This operation must not be confounded with incrustation, which is often mistaken for it, and takes place where substances, like bending twigs, have deposited upon them layer after layer of lime, salt, sulphur or ice.
The molecules of solids, even, are in intense and ceaseless motion. As has been said, “A continuous and restless, nay, a very complicated activity is the order of Nature throughout all her individuals, whether these be living beings or inanimate particles of matter. Existence is, in truth, one continued fight, and a great battle is always and everywhere raging, although the field in which it is fought is often completely shrouded from our view.”
Ex.—A simple illustration of the convenience of machinery in applying force and changing direction.
The motto of the brave Huguenots in the time of Louis XIV. was “Ever burning, but never consumed.”
Nature’s motto, both for matter and energy is, “Ever changing, but never destroyed.” Let us next notice some instances of the
CONSERVATION OF ENERGY.
Energy is the power to do work or overcome resistance. It is of two kinds—potential and kinetic. The former is the energy or force due to position, but it is latent or inactive. The latter is the energy of a body which is in motion. A stone resting on a mountain top, the water in a quiet mill pond, a coiled spring, are all examples of potential energy.
The stone, crushing through the cottage of a peasant, the water turning a factory wheel, the spring turning the wheels of a clock, are examples of actual or kinetic energy.
