'98 PRINCIPLES OF CHEMISTRY. 



Atomic weight. If atoms exist, they must have weight, since 

 they are concrete masses of matter, and all matter is attracted by the 

 earth. But, of course, it is impossible to weigh single atoms, and we 

 have no knowledge of the absolute weight of an atom. It is possi- 

 ble, however, to determine the relative weights of the atoms, that is, 

 how many times heavier one atom is than another. How this is done 

 will be briefly indicated in the next chapter. In any process of 

 weighing, we adopt a unit with which to make a comparison. For 

 example, in commerce we have a mass of iron which is called a 

 pound, and we say a body weighs so many times the mass of iron, or 

 so many pounds. We proceed similarly in the case of atomic 

 weights. We use the weight of an atom of one element as the unit, 

 and compare the weight of the atoms of the other elements with it. 

 The thing to decide is, Which atom shall be chosen as the unit 

 weight ? As was said in discussing combining weights, hydrogen is 

 the lightest known substance, and it unites with other elements in the 

 smallest proportions, hence its atom is chosen as the unit weight. 

 The figures assigned to the other elements as atomic weights simply 

 signify how many times heavier those atoms are than the atom of 

 hydrogen. In other words, the atomic weights are nothing more 

 than ratios. On the page preceding the Index is a table of the ele- 

 ments, with symbols and atomic weights. 



Atoms and molecules. In Section I, on Physics, we learned that 

 all matter is made up of exceedingly minute particles, called mole- 

 cules, and, from a chemical study of matter, we are led to believe 

 that there is another kind of small particle, the atom. The atoms 

 unite in chemical action and form the larger masses called molecules. 

 The molecules of compounds consist of atoms of different kinds. 

 The elements also, like any kind of matter, consist of molecules, 

 which evidently must be made up of atoms of the same kind. There 

 are a few exceptional elements of which the molecule consists of only 

 one atom, that is, the molecule and atom of these are identical. The 

 atoms of an element have the power of uniting not only with atoms 

 of a different kind, but also with themselves, to form molecules of 

 the element. The present conception as to atoms may be summed 

 up in the following definition : "Atoms are the indivisible constitu- 

 ents of molecules. They are the smallest particles of the elements 

 that take part in chemical reactions, and are, for the greater part, in- 

 capable of existence in the free state, being generally found in com- 

 bination with other atoms, either of the same kind 'or of different 

 kinds" (Remsen). 



