4 PHYSICS, 
Properties of Matter. 
Under the name of Matter is included everything that we become 
acquainted with by means of our senses. Farther on, Matter will be 
treated of under the forms, Solid, Liquid, and Gaseous; but there are 
certain properties, common to all kinds of matter, which must first be 
described. 
1. Extension or Magnitude.—Extension or magnitude is the property of 
matter which implies that it 7s extended or occupies room or space. 
Bodies are extended in three directions, or have three dimensions or 
measures—length, breadth, and depth. Width is another term used for 
breadth ; and for depth we often use height, and sometimes thickness, 
By these three dimensions the shape of a body is determined. If we think 
of a stone, it may be round or square; or if of a mountain, it may be 
high or low; but it will have some shape. When we speak of the size of 
anything, we can do so only by comparing it with something else, the size 
of which we do know; for example, a boy describes something to his friend 
as being as big as his fist, or as big as his head. For the sake of convenience, 
standards of measurement have been fixed upon to be used by all. For 
length, the inch is the standard ; and we say a thing is so many inches or 
so many feet long. For measuring a surface, which has both length and 
breadth, the standard is a small square an inch long and ah inch broad, 
called a square inch; and a surface is said to contain so many square 
inches, or so many square feet. Lastly, for solids, which have length, 
breadth, and depth, the standard is a small cube, each side of which isa 
square ak and the bulk or volume of any quantity of matter is said to 
be so many alia inches or so many cubic feet. 
2. Impenetrability—The word impenetrability must have a sonia 
meaning here, for there is no material so hard that it could not be 
penetrated or Bicwol; if proper instruments were used. Impenetrability, 
as applied to matter, means simply that two bodies cannot be in the same 
place at the sume tume. A nail can be driven into wood, but it is impos- 
sible that there can be wood in the very space occupied by the nail: 
the particles of the wood are merely forced more closely together, in order 
to make room for it. This property of matter is obvious in regard to solid 
bodies, -but it is not so obvious with regard to fluids. Common air 
offers so little obstruction to our movements, that we are apt to forget that 
it is a real material body. That it is so, can be shewn by many simple 
illustrations. When a bladder is filled with air, it is impossible to 
press the sides together without bursting the bladder. If a tumbler 
be put, mouth downwards, into a vessel full of water, the water does not 
fill the tumbler completely, because it is prevented from doing so by the 
