109 



melanocerite and karyocerite , contain a slight amount of tant- 

 alic and phosphoric acid. 



In spite of the rather considerable differences in the com- 

 position of the single minerals some features may be mentioned 

 common to all of or most of the minerals. Of these are above 

 all to be mentioned the frequent occurrence of rare elements, 

 partly of uranium, thorium, and zirkonium, and partly of the 

 cerium metals; both kinds are only wanting in ardennite, epi- 

 stolite and the antimonat^s. Besides the cerium metals there 

 are of basic ingredients always found CaO or FeO, or both, 

 and frequently NcCoO; on the other hand only exceedingly 

 small amounts are found of all other bases , especially MgO 

 and K» are almost never present except in quite minimal 

 amounts. The content of water is generally not large , and 

 may, perhaps with the exception of epistolite, be owing to a 

 beginning transformation, it is a rather peculiar fact that in 

 all the niobates no tantalic acid is found, or it is at all events 

 only found in very slight quantity, although these two elements 

 are otherwise always found together. 



It is especially difficult to establish an exact classification 

 of these minerals , as they are almost all of them of a very 

 complicated chemical composition, and it is scarcely possible 

 with regard to any one of them to state a certain formula of 

 composition. With regard to the minerals of which several 

 analyses have been made, these analyses differ rather much 

 from each other, and where only a single analysis is found, a 

 reliable formula can scarcely be made from it. 



With regard to the minerals having a single acetous in- 

 gredient, or, at all events, different acids of the same atomicity, 

 the most common base of division is their oxygen ratio, i. e. 

 the ratio between the atoms of oxygen connected with the basic 

 ingredients, and those connected with the acetous ones. When 

 this ratio is calculated for the minerals with different acids 

 mentioned here, we shall find, with quite few exceptions that 



