THE DOMAIN OF PHYSICS. 3 



5. What is a Molecule IA molecule is the 

 smallest quantity of matter that can exist by itself. 



Molecules are exceedingly small, far beyond the reach 

 of vision even when aided by a powerful microscope. 



(a.) We know that a drop of water may be divided into several 

 parts, and each of these into several others, each part still being 

 water. The subdivision may be carried on until we reach a limit 

 fixed by the grossness of our instruments and vision ; each particle 

 still is water. Even now, imagination may carry forward the 

 work of subdivision until at last we reach a limit beyond which we 

 cannot go without destroying the identity of the substance. In 

 other words, we have a quantity of water so small that if we divide 

 it again it will cease to be water ; it will be something else. This 

 smallest quantity of matter that can exist by itself and retain its 

 identity is called a molecule. The worcj molecule means a little 

 mass. (See Avery's Chemistry, 4.) 



(5.) The smallest interval that can be distinctly seen with the 

 microscope is about VV ^ VT inch. It has been calculated that about 

 2000 liquid water molecules might be placed in a row within such 

 an interval. In other words, an aggregation of 8,000,000,000 water 

 molecules is barely visible to the best modern microscopes. 



6. What is an Atom ? An atom is the 

 smallest quantity of matter that can enter into 

 combinatioii. 



In nearly every case an atom is a part of a molecule. 



(a.} If a molecule of water be divided, it will cease to be water 

 at all, but will yield two atoms of hydrogen and one of oxygen. 

 The molecule of common salt consists of one atom of sodium and 

 one of chlorine. Some molecules are very complex. The common 

 sugar molecule contains forty-five atoms. 



(&.) Atoms make molecules ; molecules make masses. Of the 

 absolute size and weight of atoms and molecules little is known ; 

 of their relative size and weight much is known, and forms an im- 

 portant part of the science of chemistry. 



7. Forms of Attraction. Each of these three 

 divisions of matter has its own form of attraction : 



