274 CHEMICAL DISCOVERY AND INVENTION 



that of an ordinary flame. The well-known limelight, in fact, 

 requires the use of oxygen with the gas to produce the necessary 

 temperature. It was only toward the latter end of last century 

 that serious attempts began to be made to utilise the earlier 

 observations of Berzelius and Bunsen on the peculiar incan- 

 descence produced by several of the rare earths. It is un- 

 necessary to trace the various attempts to utilise the incan- 

 descence of magnesia, zirconia, and other substances, for the 

 discoveries made by Dr. Carl Auer, 1 as a consequence of his 

 studies of the rare earths begun about 1885, require all the spacf 

 which can be spared for this subject. The " mantle," which is 

 familiar in almost every household where gas is the illuminant, 

 consists of a mixture of thorium oxide with about 1 per cent of 

 cerium oxide. The use of this mixture was the result of a long 

 series of trials, and was protected by patent in 1893. Since this 

 discovery minerals which contain thorium have become very 

 important and valuable. 



At one time they were known chiefly as of Swedish origin and 

 were even called the Swedish earths. But the immense quantities 

 now required are supplied from monazite sands found in extensive 

 deposits in N. and S. Carolina and especially on the coast of 

 Brazil. Other important minerals are thorianite, an oxide very 

 rich in thoria, found in Ceylon, and thorite, a silicate which occurs 

 in various localities in Scandinavia. Monazite, which is essen- 

 tially a phosphate of cerium containing relatively small quantities 

 of thorium, is the chief material now used in connection with the 

 mantle industry. 



Its composition is, however, very complicated, and the 

 extraction of the small percentage of thorium present is a matter 

 of difficulty. 



In dealing with Brazilian monazite sand the first operations 

 are mechanical, and advantage is taken of the high specific 

 gravity of the mineral (about five times heavier than water) to 

 remove by streams of water much of the lighter material. 

 Electro magnets are also used for extracting ferruginous particles, 

 and a concentrated material is ultimately arrived at which retains 

 only 2 or 3 per cent of impurity. The mineral is then treated 

 with sulphuric acid for the extraction of the earths. 



Here, however, we may halt, for an exposition of the details 

 of the process to be followed would be extremely tedious 

 1 Now known as Baron Auer von WeJsbach, 



