MATTER AND ITS PHYSICAL PROPERTIES. 



21 



The quantity of space which a body occupies is sometimes called its magni- 

 tude. 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 ap- 

 plication 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. 



IMPENETRABILITY. 



This property will be most clearly explained by defining the positive quality 

 from which it takes its name, and of which it merely signifies the absence. A 

 substance would be penetrable if it were such as to allow another to pass 

 through the space which it occupies, without disturbing its component parts. 

 Thus, if a comet, striking the earth, could enter it at one side, and, passing 

 through it, emerge from the other without separating or deranging any bodies 

 on or within the earth, then the earth would be penetrable by the comet. 

 When bodies are said to be impenetrable, it is therefore meant that one cannot 

 pass through another without displacing some or all of the component parts of 

 that other. There are many instances of apparent penetration ; but in all these 

 the parts of the body which seem to be penetrated are displaced. Thus, if the 

 point of a needle be plunged in a vessel of water, all the water which previ- 

 ously filled the space into which the needle enters will be displaced, and the 

 level of the water will rise in the vessel to the same height as it would by pour- 

 ing in so much more water as would fill the space occupied by the needle. 



FIGURE. 



If the hand be placed upon a solid body, we become sensible of its impene- 

 trability, by the obstruction which it opposes to the entrance of the hand within 

 its dimensions. We are also sensible that this obstruction commences at cer- 

 tain places ; that it has certain determinate limits ; that these limitations are 

 placed in certain directions relatively to each other. The mutual relation which 

 is found to subsist between these boundaries of a body, gives us the notion of 

 its figure. The figure and volume of a body should be carefully distinguished. 

 Each is entirely independent of the other. Bodies having very different vol- 

 umes may have the same figure ; and in like manner bodies differing in figure 

 may have the same volume. The figure of a body is what in popular language 

 is called its shape or form. The volume of a body is that which is commonly 

 called its size. It will hence be easily understood that one body (a globe, for 

 example) may have ten times the volume of another (globe), and yet have the 

 same figure ; and that two bodies (as a die and a globe) may have figures alto- 

 gether different, and yet have equal volumes. What we have here observed 

 of volumes will also be applicable to lengths and areas. The arc of a circle 



