ATOMS AND MOLECULES 43 



might divide it into a particle so small as to be 

 quite invisible to the most potent microscope, yet 

 it would still be a particle of mercury. Eventually, 

 if we continued dividing, we would reach a particle 

 which could be no further divided ; and even this 

 final particle would still be mercury. Substances 

 of this kind, which can be divided and divided 

 without change of character down to a final in- 

 divisible, invisible particle (as contrasted with a 

 substance like water or salt, which can be divided 

 into diflferent constituent substances), are known 

 as elements^ and the ultimate indivisible particles are 

 known as atoms. Further, the atoms of diflferent 

 elements have been shown to have different 

 weights, known as their atomic weights. The 

 lightest atom is the atom of the gas, known as 

 hydrogen, and with it the weight of all other atoms 

 is compared. Thus the atom of sulphur, which 

 is twelve times as heavy as an atom of hydrogen, 

 is said to have an atomic weight of 12. 



The process of reasoning by which we reach 

 these facts need not be detailed here. Suffice it to 

 say that the facts are fully established, and that 

 there are known to be about eighty elements, each 

 with atoms of a particular atomic weight. 



Atoms have a strong aversion to single life ; 

 in fact they never remain single, and are always 

 found in twos or threes, or occasionally in hundreds 



