526 BARON A. E. NORDENSKIOLD ON THE OCCURRENCE [Aug. 1 90O, 



sible. But in 1876 I found that thorite occurred more abundantly 

 than at Brevig in some pegmatite-veins in the neighbourhood of 

 Arendal; and two years previously, on examining the minerals 

 associated with the gold which is washed in Finnish Lapland, I 

 had ascertained that monazite occurs there in comparatively large 

 quantity. More recently, thorite has also been met with in fair 

 abundance at several new localities in Southern Norway, 



Thoria, mixed with slight quantities of other rare earths, is now 

 largely and profitably utilized for the production of the Welsbach 

 incandescent light. At present, probably a hundred thousand 

 Welsbach lamps, or, as they might be called, thoria-lamps, are 

 in use in Sweden alone ; and for these lamps about 100 kilogrammes 

 of thoria-salts are required every year. As a consequence of this 

 4 thoria-ores ' are in great demand ; they were for some time the 

 object of a lucrative export from the felspar-quarries of Southern 

 Norway. This export, however, ceased after it was found that the 

 sand of several river-beds in the Brazils and South Carolina, like 

 the auriferous sand at Ivalo in Finland, contains so large a per- 

 centage of monazite that it will pay to ship this sand to Europe, 

 where the thoria contained in the monazite is extracted. 



In 1785 Cavendish, when analysing air, first by the admixture of 

 oxygen and the oxidation of the nitrogen with the aid of the electric 

 spark, and then causing the oxygen to be absorbed by appropriate 

 means, found that a small gas-bubble, which could not be destroyed 

 in this way, always remained in the eudiometric tube. As we 

 now know, this gaseous residue consisted of the substance, discovered 

 more than a century later by Rayleigh and Bamsay as a hitherto 

 unknown constituent of the air, and named argon. 



In argon small percentages of two more gaseous elements, 

 crypton and neon, have since been discovered, to which possibly 

 a third, me tar g on, will have to be added. Thus, the close and 

 careful examination of the infinitesimally small gaseous residue in 

 Cavendish's eudiometer has proved wonderfully rich in scientific 

 results. Berzelius, when analysing thorite (1829), had also observed 

 that the mineral, when heated, yielded, besides water and a little 

 carbon-dioxide, nearly -5 per cent, of an inert gas. A closer exa- 

 mination of this gas, however, could not be made by him, owing to 

 the scanty supply of material available for analysis. This gaseous 

 residue probably consisted of a mixture of nitrogen and helium, 

 that is, of nitrogen and the element which, discovered several years 

 ago in the envelope of the sun by means of spectrum-analysis, was 

 named helium by Frankland and Lockyer, and has since been 

 discovered by Bamsay in several terrestrial minerals, chiefly in such 

 as contain rare earths. 



It was not, however, from thorite that helium was first isolated, 

 but from another mineral containing thoria. This mineral was 

 discovered by me in 1877 among specimens from the quarries near 

 Arendal, and was named cleveite. The same mineral was found 

 a few years later by Prof. Brogger among samples from the felspar- 



