A CENTURY OF CHEMISTRY. 73 



undecomposable substance — since we must always 

 bear in mind that an increased perfection of method 

 may result in the decomposition of what was pre- 

 viously regarded as elementary. 



Recent Discoveries of New Elements. — During 

 the last quarter of a century the number of known 

 elements has been very rapidly increased. In a gen- 

 eral way, it may be said that analysis has become 

 more penetrating, but there are several particular 

 reasons for the increase. (1) It was by the electro- 

 lytic decomposition of alkaline earths that Davy dis- 

 covered potassium and sodium ; this was about the 

 beginning of the century, and the discoverer had at 

 his command only a feeble Voltaic pile; now in- 

 tensely powerful currents are utilised, and it was by 

 these that Moissan, for instance, was able to isolate 

 fluorine from its combinations. (2) Spectrum 

 analysis has shown the existence of a series of ele- 

 ments with characteristic spectra, and it is a remark- 

 able fact that one of these, helium, was known from 

 the sun before it was discovered in the earth. (3) 

 Certain theoretical conceptions, such as Mendelejeff^s 

 periodic law, have led chemists to look out for and to 

 find elements whose existence was predicted on a 

 priori grounds. Thus Nilson in 1879 discovered 

 scandium which Mendelejeff had foretold. Gallium, 

 discovered by Lecoq de Boisbaudran in 1875, and 

 germanium, discovered by Winkler in 1886, are other 

 famous examples. 



Argon. — Two of the latest additions to the list of 

 elements deserve special notice. In 1892, Lord 

 Rayleigh directed attention to the fact that nitrogen 

 obtained chemically was about one-half per cent, 

 lighter than that got from the air, and it was this 

 minute discrepancy which led him to look for and 



