530 MINERALS CONTAINING RARE ELEMENTS. [Aug. I9OO,- 



that enters into these minerals always consists of a mixture of several 

 oxides, so too the inert gas which the mineral yields seems to 

 contain not one but several closely allied gases. The group of earths 

 as well as the group of gases of which we are here speaking might, 

 therefore, be compared with certain genera among organic beings 

 whose species, not having yet been fully differentiated, present to the 

 descriptive zoologist or botanist difficulties analogous to those with 

 which the chemist meets in trying to separate the rare earths and 

 the rare gases. 



As fergusonite is a compact substance free from such pores or 

 minute cavities as occur so abundantly in many minerals, the helium 

 in it must, as Prof. Ramsay observes, be in some way chemically 

 combined. As yet, however, we have no more tangible idea of the 

 nature of that combination than we have of the mysterious con- 

 nexion that seems to exist between the rare gases and the rare 

 earths. Some philosophers have tried to give an explanation of 

 this, by assuming that the pegmatite has broken forth as a glowing 

 melted mass or a heated magma from the interior of the earth ; and 

 that helium is exclusively bound to the minerals of the pegmatite, 

 on the ground that this gas, which (owing to its slight molecular 

 weight) could not in its free state remain in our atmosphere, has 

 been imprisoned in the interior of the globe on account of the high 

 pressure there prevalent. This explanation is, however, quite un- 

 tenable, being based on an erroneous assumption concerning the 

 nature of the pegmatite-veins that contain the rare minerals. These 

 viens thin out at an inconsiderable depth below the earth's surface ; 

 they have no connexion with a solidified granitic magma in the 

 interior of the earth ; and the minerals that contain the rare earths 

 are mainly confined to the flucans alongside the pegmatite-veins, 

 which are, like calcite-veins in the older rocks, geologically speaking, 

 children of the latest birth. In many places they even appear to be 

 still in course of formation. 



The presence, therefore, of helium and rare earths in the minerals 

 found in these veins is still, like many other facts connected with 

 the mode of occurrence and the chemical constitution of the- 

 recently discovered gases and the rare earths, a mystery to the 

 student free from an uncritical predilection for antiquated theories 

 and hypotheses. 



Discussion. 



Prof. Miers remarked on the isolated occurrence of the rare 

 elements in certain patches of the earth's surface, and deplored the 

 ignorance which at present prevails concerning their distribution in 

 Cornwall, where rhabdophane, churchite, and monazite have been 

 discovered : monazite he had himself found in the slate at Tintagel. 

 The interesting historical survey given in the present paper directed 

 attention to the importance of the mineralogical distribution of 

 these rare elements. 



The President also spoke. 



