The Romance of Chemistry 737 



always changing into something else; or it may be that they are 

 being made from other elements which are also rare. A signifi- 

 cant fact is that the dozen rare elements mentioned above are 

 found associated together as if they formed a family party. 



Let us pass to something easier man's use of some of the 

 rarities. Towards the end of the nineteenth century, Carl von 

 Welsbach hit upon the .brilliant idea of increasing the incandes- 

 cence of a gas flame by making it heat up a cotton mantle im- 

 pregnated with a rare element, such as lanthanum. Various 

 improvements followed, but the principle remains the same to- 

 day. The cotton mantle was replaced by a fabric made of china- 

 grass, and the lanthanum was replaced by 99 per cent, thoria and 

 1 per cent, of ceria two rare earths which work into one another's 

 hands in some rather mysterious way. The theory is obscure ; the 

 practical result is brilliant light. 



In 1897 Professor Nernst of Gottingen showed that a fila- 

 ment made of rare earths (thoria or zirconia) was at ordinary 

 temperatures a non-conductor of electricity, but a very good one 

 when heated even when "lighted by a match." So we got the 

 Nernst electric lamp, which gives a far finer light than the ordi- 

 nary carbon filament electric lamp, but has the disadvantage of 

 various complications and delicacies. Other rare elements have 

 been tried, each with its own virtues. Thus there is a lamp with 

 a filament of the very rare osmium, "probably the most refractory 

 and unalterable solid known to science," and therefore furnishing 

 a filament with a very long life. There is another with a filament 

 of the unrustable, steel-hard, element called tantalum, which oc- 

 curs in South Dakota and in Australia, as part of the mineral 

 called columbite. Still more recent, and also very efficient, is the 

 lamp with filaments of the rare metal tungsten. It must be noted 

 that there are many effective lamps, such as the carbon-lamp, 

 which do not use rare earths; but it is of interest to realise that 

 some of the rarities among the elements are common in the lamps 

 of our houses and streets, and that academic curiosities have be- 



