EXTENSION, in philosophy, one of the general and essential properties of matter; the extension of a body being the quantity of space which the body occupies, the extremities of which limit or circumscribe the matter of that body. It is otherwise called the magnitude, or size, or bulk of a body.

A quantity of matter may be very small, or so its to elude the perception of our senses, such as a particle of air, a particle of water, &c.: y et some extension it must have, and it is by the comparison of this extension, that one body is said to be larger than, eqnal to, or smiler than, another body. The measurement of a body consists in the comparison of the extension of that body with some determinate extension, which is assumed as a standard, such as an inch, a foot, a yard, a mile; hence it is said, that a body is a foot long, or three inches long, &e.

The extension of a body is measured three different ways; or a body is said to have length, breadth, and thickness. Titus an ordinary sheet of writing paper is about 16 inches lung, about 1-1 inches broad, and nearly one hundredth part of inch thick. Either of these dimensions might be called the length, or the breadth, or the thickness; lint, by general custom, the greatest extension is called the length, the next is called the breadth, and the shortest is called the thickness. The outside of a body, its boundary, or that which lies con tiguous to other bodies that are next to it, is called the satfure of that body, and this sudace has two dimensions only, viz., length and breadth ; but it has no thickness, for if it had, it would not be the outside of the body ; yet at surface by itself cannot exist. In mathematics, however, surffices are mentioned, and are reasoned upon, abstractedly from matter. But in these eases the surfaces exist in the imagination only, and even then our ideas have a reference to body, for our senses cannot perceive a surfiice without a body.

As a surface is the outside or boundary of a body, so a line is the boundary of a finite surface. Suppose, for instance, that

a suffice is divided into two parts, the common boundary of the two parts is called a line ; this has one extension only, viz., it has length.

The beginning or the end of a line, or the intersection of two lines which cross each other, is called a point, and this has no dimensions; or, according to the mathematical defi nition, a point is that which has no parts or magnitude, its use frequently is to mark a situation only as a point upon a surface by the intersection of two lines, &c. Thus, if you divide a line into two parts, the division or boundary between the two parts is a point.

Our senses are only capable of perceiving bodies which have three dimensions; or rather the surffiees of bodies, which surfaces have two dimensions, but a surface cannot be represented nor perceived without at body, and of course neither a line nor a point can be perceived without a body. In the study of geometry, and in a variety of other branches, surfitces, lines, and points are represented upon paper, or upon something else; but in those cases, the paper, or that something else, is the body whose surface we 'Perceive, and the surface of a particular figure is circumscribed, not by real lines, but by a narrow slip of surffiee, which is sufficient to direct our reasoning with respect to the geometrical pro perties of lines and surfaces. Thus also, when points are represented by themselves, the marks are not real points, but very small portions of the su•f:ice of a body.

There is a ease in which extension is often said to be per ceived without the existence of a body, and this is the exten sion between two bodies. But, upon consideration, it will easily be comprehended, that we may perceive the two bodies, and that they arc separate from each other ; but we cannot perceive anything positive between them. So that in this case the word extension is used in a figurative manner, as if some other body existed between the two bodies.