PRESIDENTIAL ADDRESS. 15 



92. The great regularities noticed at first between the 

 properties of elements, like the halogens or the alkali 

 metals, have resulted in the construction of one of the 

 most interesting and useful diagrams available to the 

 chemist; the use and interest remain, but it will not be 

 possible to be content with the Periodic table until the 

 great irregularity within the series of regularities is better 

 understood. 



Combination of elements to form compounds. 

 From the elements are obtainable, by combination, com- 

 pounds; and only such compounds have arisen in nature as 

 have not been unstable under the conditions obtaining for 

 the time being. We can, however, prepare in the laboratory 

 many compounds not found in nature. Each molecule of 

 a compound is an aggregate of elementary atoms. The 

 atom of an element retains unaltered its mass and, if radio- 

 active, its radioactivity, when the atom enters a compound. 

 All other properties are however liable to be altered; for 

 example, the atomic volume is altered. The properties of 

 a compound are not the mean of the properties of the 

 composing elements. A student beginning to study 

 chemistry must surely be impressed by the fact that 

 whereas sodium and chlorine are separately extremely 

 poisonous substances, sodium chloride, formed by burning 

 sodium in chlorine, is essential to life. 



The change in properties exhibited by an element when 

 its atoms become ionised is very great. Ferrous ion differs 

 markedly from metallic iron and from Ferric ion. The 

 Ferrous ion is an iron atom with two positive charges of 

 electricity, (or an iron atom minus two electrons from the 

 outer shell, each electron being a unit of negative elec- 

 tricity). The Ferric ion carries three positive charges of 

 electricity. The transformation Fe ++ "^ > Fe + + + is easily 

 carried out, and yet the difference between the properties 



