310 Royal Institution : — Prof. Roscoe on Vanadium, 



line form of two minerals, viz. apatite (a phosphato-fluoride of cal- 

 cium), and pyromorphite (a phosphato-chloride of lead), to which 

 may be added mimetesite (an arsenato-chloride of lead). These mi- 

 nerals all have an analogous composition, being represented by the 

 formulae 



Apatite 3 (Ca 3 P 2 O 8 ) + CaFl 2 



Pyromorphite.. 3(Pb 3 P 2 8 ) + PbCl 2 

 * Mimetesite.... 3(Pb 3 As 2 8 ) + Pb CI 2 . 



They are truly isomorphous, crystallizing in hexagonal prisms, ter- 

 minated with hexagonal pyramids, having the same angles and the 

 same length of axes. Rammelsberg added to this list the mineral 

 vanadinite, which he ascertained by measurement to be strictly iso- 

 morphous with the foregoing, and to be as follows : — The angle P 

 on P was in 



(1) Vanadinite.. 142 30 | (3) Pyromorphite. 142 15 



(2) Apatite 142 20 | (4) Mimetesite . . 142 7 



and the relation of the lengrth of the axis, 



(1) 1:0-727 



(3) 1 : 0-736 



(2) 1:0-732 



(4) 1 : 0-739 



So far, indeed, has the identity of crystalline form been traced, 

 that crystals have been found which at one end consisted of vanadi- 

 nite, and at the other of pyromorphite (Heddle). Now, judging from 

 the crystallographic analogies alone, we shall conclude that the for- 

 mula of vanadinite is 



3(Pb 3 V 2 8 )+PbCl 2 f, 



the oxide of vanadium contained in the mineral having a formula 

 V 2 O 5 , agreeing with the corresponding oxides of phosphorus and 

 arsenic, P 2 O 5 and As 2 O 5 . In making this assumption, however, 

 we are at once confronted with the unyielding chemical facts of 

 Berzelius, according to which the oxide in question must be repre- 

 sented by the formula V 0\ and contains three, and not five, atoms 

 of oxygen. 



It is evident, then, that we have here either to do with an excep- 

 tion to the law of isomorphism, or else Berzelius's views are erro- 

 neous. Until this latter has been proved to be the case, however, 



* This group of minerals may be considered as calcium triphosphofluor- 

 hydrine, &c, thus : 



3(PO'")i Q9 3PO'") Q9 



Ca 5 f U Ca 5 f ° 



Fl CI 



Apatite. Pyromorphite. (Wiirtz.) 



t Or lead trivanadochlorhydrine, 



3VO"'( Q9 

 Pb 5 ( U 

 CI 



