6 ELEMENTARY AGRICCTLTURAL CHEMISTRY 



of copper, and, like almost all compounds, was formed from its 

 constituents with the evolution of heat. 



A consideration of the meaning of the term, atom, will clearly 

 show that it cannot be applied to even the smallest conceivable 

 portion of water or other compound substance, since such a 

 portion of matter must contain at least two atoms, and there- 

 fore can be further subdivided. Another term, molecule, is 

 therefore used for the smallest conceivable portion of a sub- 

 stance which can exist alone. Tndeed, even with elementary- 

 substances it is usually the case that separate atoms do not 

 exist, but that the molecules of elements contain two or more 

 atoms. Thus, free oxygen exists as molecules, each containing 

 two atoms, and though molecules composed of three atoms of 

 oxygen are known, they are entirely different in their properties 

 from ordinary oxygen, and constitute the substance known 

 as ozone. It is only in the case of some few elements that 

 separate atoms exist in the free state. One of the best 

 examples is afforded in the recently discovered element, argon. 

 This substance consists entirely of separate atoms, so that in 

 this and similar cases the molecule and the atom are identical. 

 When chemical action takes place no creation or destruction 

 of matter occurs, but new molecules are formed by some re- 

 arrangement of the elements present. The chemist endeavours 

 to investigate fully the changes which take place, and in many 

 cases is able to represent the whole of the changes which occur 

 by means of chemical equations. To take a simple case, con- 

 sider the union of hydrogen with oxygen to form water. In 

 hydrogen gas there are only molecules of hydrogen, each 

 molecule represented by the formula H*, while in oxygen there 

 are only molecules, Oa. When union occurs, two molecules 

 of hydrogen unite with one molecule of oxygen to form two 

 molecules of water. This can be represented thus : 



2Ha + Oa = 2H2O. 



A chemical equation is like an algebraical equation, in that 

 there must be the same quantities of each element on each side 



