LESSONS IN PHYSICS. 17 



pushed under the rim of the glass it rises to the surface of the 

 water inside the glass, pushing out a bubble of air, showing that 

 air and wood or cork can not occupy the same place at the same 

 time. These experiments may be varied in many ways with profit 

 and pleasure, and will fully satisfy the experimenter of the fact 

 that matter occupies space and possesses the properties of exten- 

 sion and impenetrability. 



The property of matter, which allows it to be divided into small 

 particles, is called divisibility. We may crush a piece of common 

 salt to powder, and dissolve the powder in water, when the par- 

 ticles become so small as to escape the sense of sight, even when 

 aided by the most powerful microscope, and only the sense of 

 taste or the chemist's art can detect the salt. Again, take apiece 

 of aniline (magenta) about the size of a pin-head, dissolve in a 

 few drops of alcohol, then stir into a gallon of clear water, and 

 the little particles or divisions of aniline will tinge every drop of 

 the whole mass of water. Experiments with other substances 

 show that matter may be divided into particles that are exceed- 

 ingly small, without changing its composition. 



When we have crushed and dissolved the salt the particles are 

 so small that we can divide them no further without changing 

 their nature. But the chemist can divide the smallest particle of 

 salt into two substances, one a silvery -white metal called sodium, 

 the other a greenish-yellow gas called chlorine, while the aniline 

 may be divided into the gases hydrogen and nitrogen, and 

 the solid, carbon. Those who have experimented with great 

 numbers of substances tell us that most bodies, mineral, vege- 

 table or animal, are compound bodies, like the salt or ani- 

 line, made up of two or more simpler bodies, which are called 

 elements. 



The smallest division that can be made of a compound body, 

 without breaking it up into its component parts, is called a mole- 

 cule, and the particles that unite to form the molecule, whether 

 of the same or different kinds of matter, are called atoms. The 

 atom is the smallest division that we can make of matter. We 

 may in the future learn to divide atoms, but at present they seem 



L. S. 2 



