LUMINOSITY OF FLAMES 279 



arrive at an estimate appears to be to calculate from the amount 

 of monazite sand annually employed in the manufacture of 

 thorium nitrate. From these figures it has been estimated that 

 approximately 250 million mantles are made per annum in the 

 whole world. More than half of these were produced by the 

 German Mantle Trust (the Deutsche Auer-Gesellschaft of Berlin) 

 in its various branches. This country probably manufactures 

 from 50 to 60 millions per annum, and has imported about the 

 same number from Germany. The figures of imports prior to the 

 war are as follows : 



Year. Value. 



1908 227,486 



1909 295,950 



1910 292,467 



1911 . . . . . 277,357 



1912 322,631 



1913 303,576 



Such figures supply occasion for remarking on the rivalry 

 which has existed for many years between the several systems 

 of lighting our streets and houses. When the incandescent 

 electric lighting began to make way, the gas industry appeared 

 almost doomed to extinction so far as illumination was con- 

 cerned, and great efforts were made to increase the candle-power 

 of the gas manufactured, and to improve the burners in use. 

 Then came the Welsbach mantle, and all seemed well for a time. 

 But a new difficulty arose when the supply of the Swedish earths 

 began to be exhausted, and the earliest Welsbach Company 

 came to an end. The search for thorium-bearing minerals in 

 other parts of the world, however, was soon rewarded by the 

 discovery of inexhaustible deposits of monazite sands on the 

 other side of the Atlantic. These deposits are now the basis of 

 the vast industry which has been described in this chapter. 

 Simultaneously the character of the coal-gas produced at the 

 gas works has gradually undergone considerable modification, 

 for, with the assistance to the illuminating power afforded by the 

 mantle, it is no longer necessary to furnish gas of the relatively 

 high candle-power formerly demanded. The effect of this is 

 that a larger quantity of gas can be extracted from a ton of coal 

 than was formerly possible when an illuminating power equal to 

 16 candles per 5 cubic feet was required. In fact whereas less 



