2 ELEMJEJ^TARY AGRICULTURAL CHEMISTRY 



meaning; indiyi-tiblej, and the theory is known as the atomic 

 THEORY. It is of very ancient origin, but was first applied in 

 its modern sense to chemistry by John Dalton about the 

 beginning of the nineteenth century. There are about eighty- 

 three different kinds of atoms, and a substance containing 

 only one kind is said to be an element. The whole earth, so 

 far as is known, is made up of these eighty-three (or there- 

 abouts) elements. The atoms diflfer in weight, but those of 

 any one element are alike in weight and in all other properties. 

 The properties of any substance reside in its molecules, not in 

 its atoms. Thus, there may be two substances possessed of 

 entirely different properties and yet containing exactly similar 

 atoms, but, in such cases, the molecules are different. For 

 example, ordinary oxygen, consisting of molecules each con- 

 taining two atoms of the element, is essentially different from 

 ozone, which consists of molecules each containing three atoms 

 of the element. The actual weight of an atom is so small 

 that it is practically impossible to determine it, but the 

 relative weight compared with an atom of another kind can be 

 determined with considerable accuracy. The methods used in 

 the determination of the atomic iveight of an element cannot 

 here be described. The table on p. 3, giving the names (in 

 alphabetical order) of the elements and their relative atomic 

 weights, may be useful. It is the table published annually 

 by the International Committee on Atomic Weights and gives 

 the figures to be accepted for 1920, 



The numbers are calculated on the assumption that the atom 

 of oxygen has a weight represented by 16-000. Formerly 

 it was usual to take as the unit of atomic weight that of 

 hydrogen, but for several reasons it is now preferred to 

 refer the values to the one-sixteenth of the atomic weight 

 of oxygen. After the name of each element in the table is 

 given a symbol, consisting of a capital letter with or without 

 another letter. By a symbol is indicated one atom of the 

 element referred to, and thus a symbol has a quantitative as 

 well as a qualitative meaning. 



