Page:Elementary Text-book of Physics (Anthony, 1897).djvu/98

 coexistence of these properties we are compelled to assume that bodies are composed of extremely small portions of matter, indivisible without destroying their identity, called molecules, and that these molecules are separated by interstitial spaces occupied by a medium called the ether.

These molecules can be divided only by chemical means. The resulting subdivisions are called atoms. The atom, however, cannot exist in a free state. The molecule is the physical unit of matter, while the atom is the chemical unit.

67. Composition of Bodies.—It has just been said that atoms cannot exist in a free state. They are always combined with others, either of the same kind, forming simple substances, or of dissimilar kinds, forming compound substances.

There are about seventy substances now known which cannot, in the present state of our knowledge, be decomposed, or made to yield anything simpler than themselves. We therefore call them simple substances, elements, or, if we desire to avoid expressing any theory concerning them, radicals. It is not improbable that some of these will yet be divided, perhaps all of them. We can call them elements, then, only provisionally.

68. States of Aggregation.—Bodies exist in three states—the solid, the liquid, and the gaseous. In the solid state the form and volume of the body are both definite. In the liquid state the volume only is definite. In the gaseous state neither form nor volume is definite.

Many substances may, under proper conditions, assume either of these three states of aggregation; and some substances, as, for example, water, may exist in the three states under the same general conditions.

It is proper to add, however, that there is no such sharp line of distinction between the three states of matter as our definitions imply. Bodies present all gradations of aggregation between the extreme conditions of solid and gas; and the same substance, in passing from one state to the other, often presents all these gradations.