EXPERIMENTS IN SOIL CHEMISTRY 265 



into these two things; although they cannot separate 

 sodium and chlorine into other substances, and so they are 

 called elements. 



What then is an element? There are only between 

 seventy and eighty elements known. Everything else in 

 the world is in the form of a compound. How are com- 

 pounds formed? By the chemical union of two or more 

 elements. Just to mix the elements together will not neces- 

 sarily make a compound. They must unite in definite 

 proportions. In order to know what we mean by this you 

 must know that everything in the world is made up of very 

 small particles called molecules. The molecules are so 

 small that they cannot be seen by the most powerful 

 microscope. Each molecule is made up of still smaller 

 particles called atoms. Now when a chemical union takes 

 place a certain number of atoms of one element unite with 

 a certain number of atoms of another element or elements 

 and make a molecule of a new substance which is a com- 

 pound. We know that water is a chemical compound 

 made by the union of hydrogen and oxygen. Two atoms 

 of hydrogen unite with one atom of oxygen to make water, 

 hence we use the symbol H2O to stand for water. 



Chemical unions are taking place constantly in the 

 world of nature. 



Experiment. — Moisten a piece of iron (a nail or iron 

 filings) ; place the wet iron on a piece of paper exposed to 

 the air. Examine next day. What has happened? A 

 chemical combination has taken place between the iron 

 and the oxygen in the air. In common terms we call this 

 rust. It is iron oxide, or if hydrogen has also combined 

 with the iron and oxygen, which is probable, it is iron 



