(3.) Upon reviewing those properties of bodies which the senses most frequently present to us, we observe that very few of them are essential to, and inseparable from, matter. The greater number may be called _particular_ or _peculiar qualities_, being found in some bodies but not in others. Thus the property of attracting iron is peculiar to the loadstone, and not observable in other substances. One body excites the sensation of green, another of red, and a third is deprived of all colour. A few characteristic and essential qualities are, however, inseparable from matter in whatever state, or under whatever form it exist. Such properties alone can be considered as tests of materiality. Where their presence is neither manifest to sense, nor demonstrable by reason, _there_ matter is not. The principal of these qualities are _magnitude_ and _impenetrability_. (4.) _Magnitude._--Every body occupies space, that is, it has magnitude. This is a property observable by the senses in all bodies which are not so minute as to elude them, and which the understanding can trace to the smallest particle of matter. It is impossible, by any stretch of imagination, even to conceive a portion of matter so minute as to have no magnitude. The _quantity_ of space which a body occupies is sometimes called its _magnitude_. In colloquial phraseology, the word _size_ is used to express this notion; but the most correct term, and that which we shall generally adopt is _volume_. Thus we say, the volume of the earth is so many cubic miles, the volume of this room is so many cubic feet. The external limits of the magnitude of a body are _lines_ and _surfaces_, lines being the limits which separate the several surfaces of the same body. The linear limits of a body are also called _edges_. Thus the line which separates the top of a chest from one of its sides is called an edge. The _quantity_ of a surface is called its _area_, and the _quantity_ of a line is called its _length_. Thus we say, the _area_ of a field is so many acres, the _length_ of a rope is so many yards. The word “magnitude” is, however, often used indifferently for volume, area, and length. If the objects of investigation were of a more complex and subtle character, as in metaphysics, this unsteady application of terms might be productive of confusion, and even of error; but in this science the meaning of the term is evident, from the way in which it is applied, and no inconvenience is found to arise.