40 C C. NATURAL HISTORY SOCIETY, 



(i) Early in this century Sir Humphrey Davy conceived the 

 idea of trying the effect of an electric current on substances like 

 Lime, Caustic Potash, Caustic Soda, etc. These, although sus- 

 pected by some chemists to be Compounds, had resisted all 

 attempts at decomposition, so that they were logically held to be 

 elements. 



By this new method Davy was enabled to isolate the now 

 well-known Alkali metals, together with some others— nine in all. 



(2) Another valuable means of discovery was found in the 

 spectroscope, an instrument invented in i860 by Professors 

 Kirchoff and Bunsen. 



In its simplest form it consists of a prism and two telescopes. 

 Light is allowed to pass through a narrow slit into the tube of one 

 telescope, the effect of which is merely to make the rays parallel 

 to one another when they emerge and fall on the prism. The 

 objective of the second telescope receives the rays from the prism, 

 and on looking through the eye-piece the observer, in the case of 

 white light, sees a band of colours termed the Spectrum. 



The use of this instrument in Chemistry depends on the fact 

 that every element when raised to a sufficiently high temperature 

 gives out its own rays of light, differing from those coming from 

 any other element, and these if passed through the spectroscope 

 yield a Spectrum peculiar to the particular element. This is so 

 delicate a means of detecting the presence of an element that the 

 most minute quantities may be readily perceived. Bunsen himself 

 discovered the rare element Caesium in the ash of his cigar, and 

 others, as Lithium, Didymium, ThaUium, etc. Eight in all have 

 been found by means of this instrument. 



(3) The third agent which has been used to discover new 

 elements is neither a force nor a new instrument, but merely a 

 theory, the effect of which was to stimulate chemists to search for 

 unknown elements which were predicted by its author — Mendele'eff. 



This chemist arranged the known elements in the order 

 of their atoime weights, and found that they could be divided into 

 groups of seven. 



Every element resembled markedly in its properties the 

 element seven places in front of it and seven places behind it. To 

 make his law apparent he had however to leave several gaps 

 unfilled by any known element ; these, however, Mendeleeff pre- 

 dicted would be filled with elements which had not then been 



