16 PRACTICAL LESSONS IN SCIENCE. 



The property of matter, by virtue of which it occupies space, 

 is called extension. Most arithmetics contain tables of the Eng- 

 lish and French systems of linear, square and cubical measures, 

 and they need not be given or discussed here. 



Under ordinary circumstances, matter exists in three forms or 

 conditions : Solids, as wood and stone; liquids, as water and oil ; 

 gases, as air and illuminating gas. 



Impenetrability is a property of matter which depends upon 

 extension, illustrating and emphasizing that property. By im- 

 penetrability is meant that property by which one body of mat- 

 ter occupies space to the exclusion of all other bodies of matter, 

 or that two bodies of matter cannot occupy the same space at 

 the same time. 



The truth of the above statement may be shown by trying to 

 put two solids, as a stone and a block of wood , or two stones or 

 any other two solids, in the same place at the same time. The 

 nail in the board separates the particles of wood ; does not occupy 

 the same place with them. Take a cup or glass partly filled with 

 water, and put into it a piece of lead, a little stone, or thrust 

 into it a piece of wood, and in each case the rising of the water in 

 the glass shows that the solid does not occupy the same place 

 as the liquid water; and again, if we pour more water or some 

 other liquid into the glass, it is manifest that the two liquids 

 do not occupy the same space at the same time. Fill a deep pan, 

 holding four or five quarts, nearly full of water, and put a com- 

 mon tumbler into it; the water will force the air out of the tum- 

 bler and take its place. Invert the tumber and raise it partly 

 out of the water, being careful not to let any air enter it ; then 

 through a straw or some kind of a tube blow a little air under 

 the tumbler; the air rising drives out some of the water, showing 

 that air and water can not occupy the same place at the same 

 time. The diving bell, formerly much used for work under water, 

 illustrates this principle. If another portion of air is forced into 

 the tumbler more water is forced out, showing that two bodies of 

 air can not occupy the same space at the same time. When the 

 glefls is partly filled with air, if a piece of cork or light wood is 



