172 SCIENCE AND THE HUMAN MIND 



The number of known elements has grown from the 

 twenty recognized by Dalton till now some eighty 

 different kinds of matter have been recorded. The 

 work of discovery has proceeded fitfully. When any 

 new method of research has been applied to chemical 

 problems, a new group of elements has frequently 

 come to light. The separating power of the galvanic 

 current enabled Sir Humphry Davy to isolate the 

 alkaline metals potassium and sodium in 1807. At a 

 later date, spectrum analysis showed the existence of 

 such substances as rubidium, caesium, thallium and 

 gallium ; while, in recent years, the still more delicate 

 methods of radio-activity have disclosed bodies like 

 radium and its allied family of elements. 



The atomic weights of the elements, first calculated 

 by Dalton, are constants of fundamental importance 

 in natural science. Relations between the values for 

 different elements which possessed somewhat similar 

 properties were pointed out by Prout and Newlands, 

 and in 1869 the Russian chemist Mendeleeff showed 

 that, if the elements be arranged in a table in order 

 of ascending atomic weight, there is a remarkable 

 likeness in the physical and chemical properties of 

 various groups of elements as they fall at certain 

 regular intervals in the table. An evident gap in 

 the series, which could not be filled without introduc- 

 ing confusion into the scheme, even led Mendeleeff 

 to prophesy the existence and to predict the pro- 

 perties of an element which was not discovered till 

 some time afterwards. 



This " periodic law " shows that the physical and 

 chemical properties of the elements depend on the 

 mass of their atoms. And, since it also thus makes 



